If it can help someone, I was in the same case. (no pop up ads and sponsored ads not rewarded since 13th of March to yesterday) All has been checked (Brave ads turn on, focus assist off, campaigns availables in my region “France”, VPN etc…)
For me, and member of Brave team confirmed me, my account has flagged for suspicious activities (fraudulent behaviour), so I received ads for free. We just must to unflagged your account manually by an administrator.
Perhaps your issue is the same. To confirm.
Yo.