I had the same problem, and rsapmd was not running.
I'm tired because I've tried various methods, initially I installed it on Almalinux 8, it still didn't work, I tried again on Centos 8, it still didn't work, and finally I installed it on Centos 7, it also didn't work.
Rspamd is still not running in AApanel, but on the system rspamd its running.
Until now I can't find a solution, the mail server can't be used.
Sad