what does this message mean?

I got this message from Lichess:

Accusing other players of using computer assistance or otherwise cheating is not acceptable on Lichess. If you are confident that a player is cheating, use the report button on their profile page to report them to the moderators.

Please note that repeated violation of this policy will result in loss of communication privileges.

You can read our Communication Guidelines here: lichess.org/page/communication-guidelines.

I hardly chat with anyone (exceptionally only Antichess Training) and i got this message? Is it a warning of chatban? I plead that I am not guilty and I think its really unfair.

It means you can't accuse anyone of cheating in chats or forums; you need to use the report button to report them.

If you want, you can discuss about a warning you received here: lichess.org/appeal#help-clean-warning

@Yeltcki I never accused someone...

Sometimes it's also possible to get a false warning, in these cases you can ask about it on lichess.org/appeal and if you think you haven't done anything to get such warning then the moderator team will review the warning. If it's a fair warning then they will let you know why you got it, if it's a false warning then they will make a note about it.

The warning doesn't restrict anything on your account yet.