Mistakes?

Turns out chess is complicated like that.
Engines don't teach chess, they're mostly good for pointing out the big blunders. To learn more about chess read books, play in tournaments, that sort of thing.

I've been playing against Computer at lvl 5, but sometimes when I move a piece into no immediate danger, the system says mistake and I don't understand why. Anyway to learn from this? I would show a screenshot but idk how to do that
If it is not in immediate danger, it likely allows a strong tactic that you cannot stop because you removed a defender or because you will no longer have time to stop a threat.
It can also be the other way - you could have missed a tactic seen by the computer that would have given you a material gain or much better position. The inaccuracy/mistake/blunder classifications from the computer are based on how much worse your position is rated after your move vs. what it would have been rated had you played the computer's best move.