blessings upon you.

So because it's an Mithral Chain Mail the AC will be 16 right?

Notes are incorrect, no stealth disadvantage and no str required "Str 13 Required, Stealth Disadvantage."

chainmail imposes disadvanteges on a character sheet. fix this

Chainmail that is not made of mithral will impose disadvantage on stealth checks.

DM / Rule question could an Aarakocra use this? Rules say no Medium or Heavy armor then implies it's too heavy to fly in...

not intended as a troll, just want to see people's opinion on it...

No as it's to the classification of Medium or Heavy armor. Now personally I'd allow it after they sought out a specialist armorer that could adjust it to work with the Aarakocra frame during flight.  

That's because you're thinking of prior edition versions of mithril. Mithril used to lower the weight class of an armor by one, it doesn't do that any longer. It's lighter, but still the same weight class, so too heavy to fly in. They can get a light armor made of mithril if they want that... but there'd be no need other than looking shiny.

Correct. The note above is very poorly written. It's pretty much just Chain Mail that doesn't have a strength requirement, doesn't impose stealth disadvantage, and can be worn underneath clothing.

When I'm wearing the mithral chain mail it still gives me disadvantage on DNDbeyond, how can i fixe this?

Hopefully they will just fix this, but you can homebrew an armor without it, or just ignore the error and roll without disadvantage anyway.

could I wear it if I'm not proficient in heavy armor?

Nope. Proficiency in heavy armor is still a prerequisite. 

You can wear armor that you are not proficient in, but there is a cost.  

Armor Proficiency. Anyone can put on a suit of armor or strap a shield to an arm. Only those proficient in the armor's use know how to wear it effectively, however. Your class gives you proficiency with certain types of armor. If you wear armor that you lack proficiency with, you have disadvantage on any ability check, saving throw, or attack roll that involves Strength or Dexterity, and you can't cast spells.

Haha!

Yes Killjoy, still counts as chainmail for AC 16 just lighter in weight so stealth checks aren't rolled with disadvantage

Is there a GP value? Chain Mail = 75GP so what would that go up by?