Knowing which moves are positional mistakes is a topic that is extensive and beyond the scope of my knowledge to explain...
Anyway, don't worry about positional stuff so much at your rating. Tactics are going to decide all your games
The most insidious: losing a tempo (or maybe more than one). Everything can look fine--pieces placed well, strong pawn formation, safe king, etc.--but somehow the other guy seems to have an unusually strong initiative or attack, and no matter what you do, you can't seem to equalize. Your whole position is basically weak across the board because you slid behind on the normal time line of progress.
Aside from pawn moves, what are the moves that create weaknesses in your position?
And how do you know that "this" move is a positional mistake?
Thank you