why I'm not able to challenge others?

They told me that I was charged that i
use chess computer assistance.I did not use that kind of thing and how can I prove it?

If it walks like a duck and quacks like a duck
It just might be a duck
"I did not use that kind of thing and how can I prove it?"
What kind of thing did you use? Prove it

I don't think you are forbidden from creating a new account, starting over, and playing legitimately without computer assistance.

It doesn't matter if you cheated or not, you were tagged a cheat. Which means you probably cheated. Either way I don't care.

If you do plan on using an engine in the future do so in one of the following 2 ways.

Way 1: On your cheater tagged account, so people know you are likely to use an engine.

Way 2: In casual / unrated games I'm pretty sure not 100% (pretty sure though) that using engines is not enforced on casual games. Because sometimes people like to play my engine vs your engine or me plus engine vs you, or you plus your engine vs my engine and so on... and casual is good venue for that. Human + Engine vs Human + Engine is known as "Advanced Chess"

Or make a bot account

Or stop cheating...

@lurarose

Yes, cheating controls are still in place in casual games. Here is an anonymous casual game that was forfeited because of cheating:

Guys, dont be too harsh!
OP, did you forget to close background analysis of any game while you were playing.
Common mistake, people did " Learn from your mistake or review with stockfish on your webbrowser".

Yes i had using analysis of games , but i didnt know that is forbidden or something .

you can use chess computer analysis after a game, but not during the game.