Why do I keep getting “you cannot accept this challenge at this moment?!”

My acc. is activated, it’s been a week, and I played 7+ games.

If you're trying to grab a live game on the Open Challenges graph it's likely it was already paired by the time you got to it.

It’s not true. Because after I refresh when I get this pop up, the user remains asking the same challenge.

Depends on where you're seeing this message:
1) If you're attempting to challenge someone:
Their rating seek (what their minimum and maximum rating threshold is) does not contain your rating

2) Attempting to accept a rematch:
The user may have already cancelled the rematch or went into another game

3) Trying to start a new game:
Faulty internet connection or chess.com server issues.