My server configuration: Ubuntu 20.04 - 4GB RAM - Cyberpanel with Litespeed free version
Check your main.cf (postfix file)
nano /etc/postfix/main.cf
Add or change
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
relay_domains = $mydomain
mydestinations = $mydomain, $myhostname, localhost, localhost.localdomain