why I'm not able to challenge others?

from lichess forum:
"2. Can I use an engine in casual games (or against stockfish/the puzzles/opening training)?

A: Yes. You may ONLY IF YOU LET YOUR OPPONENT KNOW YOU'RE USING ONE. If someone wants to play a computer, they can play vs. AI. They should have expectation that they're playing a human if they accept a human's challenge..."

#7
The automatic forfeit only applies if the casual game is anonymous. As there is no way to automatically detect if the opponent has given permission in a casual game between registered users, the cheat detection system has to let the game go on.

But if your opponent reports you, you'll of course get banned if you cheated, even if the game was casual.

Interesting, that makes sense. Thanks for the info.

@Vempele I don't think cheats are banned on lichess, only tagged as a computer user, and can no longer play rated or tourney, and everyone knows it's someone using an engine.

Yes, but everyone calls it a ban anyway, even the warning message about losing games on purpose.

I know the rule of 'you have to tell you are using an engine in a casual game and you won't get banned,' but how does Lichess know that there was this agreement between the players?

I have the same problem. What have I done?