I made this archdevil to be the patron for a warlock but I think I overdid the stats. Looking at what other people have made, this thing has two hundred more hp than Asmodeus. Am I wrong? Should I debuff it or keep it with 581 health snd access to all warlock spells upcast to 9th level? I need help and I don't want to embarrass myself in front of my friends who are a D&D lore nerd and an experienced DM

You cannot overdo it with a stat block. I play in very high-power-level campaigns where the DM gives personalized boons to the PCs; we'd eat this devil for breakfast. It all depends on how strong you intend your PCs to be.

Now, 

Her name is Anakis in the title, but you are calling her Aknis everywhere in the statblock.

AC: a bit too high—avoid giving more than 25 base AC to a creature. Also, "Natural and scale armor" is unclear, she either has natural armor (tough skin, fur or scales) or is wearing armor.

HP: How do you get +251 ? That's not a multiple of 5 (her CON modifier).

The formula is: Number of hit dice × (average HP given by hit die + CON modifier).

With 60d10 and a +5 CON modifier, she should have 60 × 5.5 + 60 × 5 = 630 HP (15 more than Tiamat, a CR 30 creature).

I’d recommend looking at Zariel's stat block from Monsters of the Multiverse.

Speed: "Speed 60 ft., walk 40 ft." is unclear. You don’t need to specify "walk," as that is the default speed. Since she has wings, you probably want "Speed 40 ft., fly 60 ft."

Ability Scores: They’re a bit low for an archdevil, but it’s your choice.

Skill Proficiencies: She has none, maybe give her some ?

Saving throws : WHERE IS WISDOM ? Your creature is gonna get absolutely destroyed by the first control spell she encounters. She has a 55% chance of being incapacitated by a lvl 5 wizard.

Damage Vulnerabilities: She is an archdevil—she shouldn’t be vulnerable to Radiant damage; even vampires aren’t.

Damage Resistances & immunities: Most devils resist Cold and Poison, and some are even immune to Poison. She should also have at least resistance to bludgeoning, piercing, and slashing from nonmagical attacks that aren’t silvered.

Condition Immunities: She isn’t immune to being Charmed? That's dangerous for a devil :P

Senses: How did you calculate 18 Passive Perception? 10 + WIS modifier is 14, or 10 + WIS modifier + Proficiency would be 22.

DC: You can make abilities scale off different ability scores. For example, a grappling ability could scale off her Strength and have a DC of 23, while an AoE Fear effect could scale off her Charisma and have a DC of 22.

STR DC : 23, DEX DC: 20, CON DC: 21, INT DC: 24, WIS DC: 20, CHA DC: 22

Traits

Evil Aura: It’s odd that this scales off Strength, but it’s your choice.

Immortality: This is not how archdevils usually work but that would explain how she is still alive despite her weakness to WIS saving throws

Other Stuff: At least list them for yourself.

Spellcasting: If her spellcasting ability is Charisma, her DC should be 22. Also, 12 level 9 spell slots seems excessive. She could spam Psychic Scream and stun-lock an entire party.

Legendary Resistances: She should have some :)

Actions

You forgot to give her Multiattack. Only one attack per turn is definitely too weak.

Magic Longsword: Magic Longsword: +15 to hit is correct (+7 STR +8 PROF). What does "26 target" mean? The damage should be 2d10 (1d10 for a longsword, doubled to 2d10 because she is a Large creature) + 7 (her Strength modifier). If you want to increase the damage to +8, make it a magic weapon and also increase the attack bonus.

Hellfire: Unless Hellfire is a magic weapon, this feels like a Ranged Spell Attack. That would make it +14 to hit (+6 CHA +8 PROF) with however many damage dice you want, plus +6 for her Charisma modifier.

Magic Longbow: +15 to hit is incorrect. Ranged weapon attacks use Dexterity, so it should be +12 to hit (+4 DEX +8 PROF) for 2d8 + 4 damage.

Bonus Actions

These are fine. If you really intend for Heal Self to be at will, you could consider giving her the Regeneration trait (e.g., 10 HP per turn if she has at least 1 HP). Just a suggestion—it’s fine as is.

Reactions

Hellish Rebuke: Be careful—the spell’s trigger is being damaged, not attacked. This means your creature could cast Hellish Rebuke even if an attack misses her, but not if she is damaged by an ability or spell requiring a saving throw. If this is your intention, consider creating a custom ability with a different name to avoid confusion.

Legendary Actions

She needs some :)

Quote from Antisocial_Tiefling_Akemen >>

I made this archdevil to be the patron for a warlock but I think I overdid the stats. Looking at what other people have made, this thing has two hundred more hp than Asmodeus. Am I wrong? Should I debuff it or keep it with 581 health snd access to all warlock spells upcast to 9th level? I need help and I don't want to embarrass myself in front of my friends who are a D&D lore nerd and an experienced DM

 Why does it have a statblock? Do you intend for a plot point to be that the party hunts down and kills the warlock's patron? How is that going to work? Will the warlock lose all their powers? 

You do not need a statblock for anything that you don't intend for the party to fight, gods and warlock patrons are generally so powerful that player characters have no chance of killing them so they don't even get statblocks. If the PCs attempt to fight them you make them just laugh, wave their hand and send the party tumbling through the Astral Plane or something similar.

Thanks everyone. Updates coming soon. Need to charge my laptop first.

And I also have my real life in the way of D&D beyond :(

I have made the changes you all suggested. Thanks so much and please enjoy this fun way to annoy your party when the warlock dose something stupid.

" this archdevil " is wild.(forget mys spelin) and chill

Quote from Agilemind >>

Quote from Antisocial_Tiefling_Akemen >>

I made this archdevil to be the patron for a warlock but I think I overdid the stats. Looking at what other people have made, this thing has two hundred more hp than Asmodeus. Am I wrong? Should I debuff it or keep it with 581 health snd access to all warlock spells upcast to 9th level? I need help and I don't want to embarrass myself in front of my friends who are a D&D lore nerd and an experienced DM

 Why does it have a statblock? Do you intend for a plot point to be that the party hunts down and kills the warlock's patron? How is that going to work? Will the warlock lose all their powers? 

You do not need a statblock for anything that you don't intend for the party to fight, gods and warlock patrons are generally so powerful that player characters have no chance of killing them so they don't even get statblocks. If the PCs attempt to fight them you make them just laugh, wave their hand and send the party tumbling through the Astral Plane or something similar.

 I do that exact thing with one of my player's patrons! It works so well.

BlueMeadow listed nearly everything. I just have two things to add. First, if they are a dark magic user wouldn't they be immune to necrotic, or at least resistant? Second, the hellfire attack would be magical. One reason this could be important is if any of her enemies can cast antimagic field.