Chess is fun but these chess websites are full of idiots.
If you don't want to accept takebacks in OTB games, that's fine and normal but if you're declining them during online chess, then you're a weak player. Period.
When you play online chess you must accept a takeback if your opponent sends one. I've had too many good games ruined by some idiot declining a takeback when an obvious keyboard input mistake has happened, and it ruins the whole experience of being able to play the game.
If I go to castle my king but the website thinks I only moved my king over one square instead, and when I go to correct the mistake THE WEBSITE MADE you decline the takeback, you're telling me two things:
1. You think you're allowed to make two moves in a row.
2. You think the game will continue even though I just saw you do 1.
No way, hombre. That's not how this works. If you decline, I'll send takeback again (usually) and give you a second chance to correct your mistake of not letting me correct my mistake but if you decline it again then guess what?
Game over. You lose. I win.
You surrendered the game and I automatically win by default.
Deal with it.
Only checkmate wins the game (unless you surrender), and if you think you can decide whether or not my castled king gets to happen when I go to play it, then you clearly haven't learned how this game works.
Go learn how to play chess.