Can you please provide an error message of the occ upgrade command?
I guess, you potentially just have the wrong PHP version installed. I think RHEL 8 clones set up PHP7.2 by default which is no longer supported:
dnf list installed | grep php
# ^ check for incorrect version PHP 7.2
# Only do this if you know and understand what this does.
# If there are other web applications dependend on PHP on the server
# and incompatible with PHP 7.4 they WILL break
dnf module reset php
dnf module enable php:7.4
# The upgrade command installed the correct versions for me
# but I don't know if this is the recommended process to switch appstream versions
# Perhaps check official documentation as well
dnf upgrade
dnf list installed | grep php
# ^ check for updated version PHP 7.4
RHEL 8 is supported and works, with the correct PHP version installed.
For problems with the ownCloud client, please open a separate topic in the Desktop category.