So, I'm a new DM,one of my players is VERY knowledgeable in the lore and facts of D&D. Basically, when I give them a character interaction, and they act a little bit off their alignment, they call it dumb and unrealistic, even tho i have lore as to why they are like that, but Im not sure https://tutuapp.uno/ .

Am I Dming wrong?

As DM you can change any existing lore in anyway you want to suite your campaign. 

Also something to consider, as knowledgeable of D&D lore one of your player can be, it doesn't mean his character does and you can always tell the player that its character doesn't know about any given lore or subject in your game. Or require an ability check to see it it does. This is called metagame knowledge and player are often best to check with their DM before assuming their character know something they do, especially if it relates to place, magic items, NPC, monsters etc..

You’re not doing anything wrong. You can change the personality of people, you can move the city’s location around a map. You can change anything you like. Not just bend the lore, you can snap it into pieces and throw it in the trash.

I’d suggest an out of game talk with the player explaining to them that you are going to deviate from published lore. (Don’t ask their permission, tell them how it’s going to be.) Politely, of course, but firmly. Just explain your FR is different, and give them a minute to wrap their head around the idea. Tell them it’s an alternate reality, or you’re doing it to stop anyone meta gaming, if you feel like you need to give them a reason.

Then see what happens, if they keep doing it, you might encourage them to go and find a different table that better fits their style.

Quote from zibonvil >>

So, I'm a new DM,one of my players is VERY knowledgeable in the lore and facts of D&D. Basically, when I give them a character interaction, and they act a little bit off their alignment, they call it dumb and unrealistic, even tho i have lore as to why they are like that, but Im not sure.

Am I Dming wrong?

 

 
A little? Bend, break, or utterly eliminate or replace anything you feel like. The world is yours as the DM, do whatever you want with it.

You, as DM, are free to follow or change any lore you want for your world. Like the others answering above I’m a long time DM (1979). I run a variant Forgotten Realms that has many changes from official lore (mostly forgotten realms specific lore). At the same time I’m a lore “hound” always searching for more lore to help flesh out my world. That doesn’t mean we can’t change the lore as we see fit to build the world we want. Really, we have no choice as our adventurers are going to change that lore n the process of their adventures.

This example is very much a case of where you break out the DM bat and remind them who's running the game. Granted, if you're using existing characters it's best to keep them in a similar area, particularly if they're well-known in-universe so the characters should have some expectation of what they're like, or at least give a heads up out of play that you're re-interpreting some characters so everyone is on the same page. A certain degree of meta knowledge from players is fine for filling the role of "common knowledge" the PCs all know, like don't get into staring contests with medusae, throw fire at a red dragon, or give your honest opinion on Khelben Arynson's art.

A DM, in essence, is a story writer. A writer has two things to consider:

-How do I make my story interesting to the reader (player here)

-And what is the audience I want to entertain.

There is of course a wrong way to dm. The saying "No dnd is better than bad dnd" comes to mind. But you are the one writing your story, and someone out there will want to read it. If every dm made the same story and never deviated, no one would play.

Another thing to consider is that this is a team game, not a dm game, or a player game. Your goal as a dm is to make the game as fun as possible, For Everyone. And you are a part of that. A story you want to tell, and the players want to hear. That, is the perfect game. Not one with perfect lore, or amazing story telling, but a game you and them enjoy.

That player seems to not be a good fit for you. That's fine, it happens a lot in dnd. Talk with them, and explain things. If they disagree, again, that's fine. Your players are people same as you.

I wish you the best of luck, cause god knows we need more dms in the world. And it really is a blast.

You are not doing anything wrong. The world you create operates under the RULES you choose, not those your players choose. That said, you probably don't want to tinker with the actual rules much until you have more experience but the lore, NPCs, whatever is going on in the world, even if it is a published one, is completely up to you as the DM.

However, the specific example you gave is one that should never come up:

"Basically, when I give them a character interaction, and they act a little bit off their alignment, they call it dumb and unrealistic, even tho i have lore as to why they are like that"

In 5e there is no spell to know a creature's alignment. There is no character feature that tells a character what the alignment of another creature might be. Even the Detect Evil and Good spell has nothing to do with alignment .. it tells you if there are specific creature types near by:

"For the duration, you know if there is an aberration, celestial, elemental, fey, fiend, or undead within 30 feet of you, as well as where the creature is located."

As a result, there is no way for a character to become aware of what the "alignment" for any NPC might be. In addition, even evil creatures can choose to take actions that would appear good. Perhaps a devil rescues a character from certain death because they want the character to be grateful and perhaps manage to corrupt them. Maybe a greedy shopkeeper decides to be generous in one instance because they feel empathy for a particular customer. Alignment only suggests a general approach and view on the world, it doesn't dictate individual actions.

So, not only is the player somehow acquiring knowledge about creature alignment that they should not have, they are also misapplying what alignment means. The only creatures who might be somewhat constrained by alignment would be fiends and celestials where the alignment is part of their nature. They tend to view the world a bit more in black and white through the lens of their alignment but even then it doesn't preclude effectively good or evil actions ... like a celestial who is over zealous and kills all the members of an evil cult, including the children, without thinking perhaps of possible redemption - is that a good or evil action?

Anyway, the alignment and behaviour of NPCs is completely at the discretion of the DM so you are entitled to change things if you like.