Dr. Susi Lehtola has installed a RHEL 9 package for MOPAC 22 in the EPEL repository. If you have enabled EPEL on your system, all you need to do, as root, is
# dnf install mopac
The mopac executable and parameter file are installed in /usr/bin. Therefore in the Interface Manager one should enter the correct path for mopac (/usr/bin/mopac) and the external parameter directory (/usr/bin).