As satellite:el8 module requires now satellite-maintenance:el8 when we enable satellite/capsule repo there always has to enabled maintenance repository in order to provide the maintenance module.

Only place where we say enable just satellite repo is External DB setup for a DB host
https://access.redhat.com/documentation/en-us/red_hat_satellite/6.12/html-single/installing_satellite_server_in_a_connected_network_environment/index#preparing-a-host-for-external-databases_satellite

4.13.2. Preparing a Host for External Databases

...

2. Disable all repositories and enable only the following repositories:

# subscription-manager repos --disable '*'
# subscription-manager repos \
--enable=satellite-6.12-for-rhel-8-x86_64-rpms \
--enable=rhel-8-for-x86_64-baseos-rpms \
--enable=rhel-8-for-x86_64-appstream-rpms

Then this is going to fail as satellite module cannot get maintenance module enabled (it would be provided by missing repo)
3. Enable the following modules:

# dnf module enable satellite:el8