When I run:
sudo dnf dist-upgrade --releasever=28 --enablerepo=updates-testing
Problém 3: package subscription-manager-rhsm-1.21.1-1.fc27.x86_64 requires subscription-manager-rhsm-certificates = 1.21.1-1.fc27, but none of the providers can be installed
- subscription-manager-rhsm-certificates-1.21.1-1.fc27.x86_64 does not belong to a distupgrade repository
- problem with installed package subscription-manager-rhsm-1.21.1-1.fc27.x86_64
This is because subscription-manager-rhsm no longer exist. But you cannot just stop building the package. Either one of your package should obsolete it or ask maintainer of package `fedora-obsolete` to add Obsoletes of your package.
*** This bug has been marked as a duplicate of bug 1564735 ***