I know this should be an option in the game. The game will even tell you that. But where is that option? How do I do it? I swear I looked everywhere, but I just couldn't find the answer. Can someone please give me step-by-step guide to fix lost tooth?

'Fix' how, what do you mean? Are your kids not losing their teeth? Or Are you trying to make the visible gap go away? To give you a step by step guide on how to fix the problem, we need to know the problem

It's the latter. Fix the gap and get rid of the negative moodlet (without cheating, to be even more clear). I swear I tried everything, clicking on my Sim, on mirror, on bed, on the negative moodlet, even on the Tooth Fairy certificate, but nothing. I also tried through a parent, but also nothing. I even disabled my mods to make sure there is no conflict, but still nothing. I am really lost here and can't find the interaction.

Apparently your sim will grow a new tooth in a few days. I've not played this EP myself but some info here:
https://simscommunity.info/2023/05/...-4-loose-teeth/

Yeah, but there should also be a way to fix the tooth. The game even tells you that after losing tooth. And I saw Simmers saying they are doing that. It was even recommended to do that before aging to avoid the glitch changing Sim's appearence drastically, though that should be fixed now, but I don't want to take chances. And that's why I would like to know how to fix lost tooth.

Well, according to that link I gave you, you can wiggle the tooth before it falls out, and then put it under their pillow, but once the tooth has actually fallen out, there is nothing to do.

That's not what the message after losing tooth says. That massage clearly says there is a way to fix the lost tooth. Plus, I saw Simmers saying they did just that. But how, I have no clue. I really hope someone will help me to figure this out.

AFAIK there's nothing to "fix" about it. The tooth gap is an intended feature. Are you by chance referring to the visual bug that breaks the sims's skin that was *allegedly* fixed already? But I'd totally believe if it wasn't? I'd say patch your game, and make a save before your sim ages up just in case. Worst case scenario and ageing up with the tooth gap actually breaks your sim, you can install weerbesu's "UI Cheats Extension" to decrease your sims' age by a day or two so they'll have grown a new tooth by the time they age up. Make sure to age them up with a cake as soon as the tooth grows so they don't lose another.

Since there is this mod: https://www.curseforge.com/sims4/mo...x-missing-tooth (which I don't have installed, mind you) I am pretty sure there is "fix missing tooth interaction", because why would someone bother to create mod to remove something that's not in the game? I just need to figure out how that interaction works and where exactly it is.

Weird, I've never seen this interaction before in my life. But if a mod maker has made a mod to remove the interaction and says "This is silly and should not exist" it's probably safe to say it isn't necessary, and you may carry on your game as normal whilst ignoring it. Since I have no idea what interaction you're talking about, I can't help you any further.