Not sure why this did not make it into the release notes for 3.17 (or any docs at all), but the error message is pretty clear. Foreman 3.17 does not support Puppet 7 anymore, but I could not find any “official” information beyond the PR: Drop Puppet 7 support by bastelfreak · Pull Request #1043 · theforeman/foreman-installer · GitHub
There is however an existing thread on the forum with some information on how to update: Switch to puppet 8
