When attempting to upgrade from Fedora Core 27 to Fedora Core 28, it may fail on the nss-pem package.
I spent some time trying to find the solution for this, and came across numerous posts on the “Red Hat Bugzilla”, particularly this post.
See below example:
[root@SYSTEMZ01 ~]# dnf system-upgrade download --releasever=28 Before you continue ensure that your system is fully upgraded by running "dnf --refresh upgrade". Do you want to continue [y/N]: y Fedora 28 - x86_64 - Updates $ Fedora 28 - x86_64 $ google-chrome $ RPM Fusion for Fedora 28 - Free - Updates $ RPM Fusion for Fedora 28 - Free $ RPM Fusion for Fedora 28 - Nonfree - Updates $ RPM Fusion for Fedora 28 - Nonfree $ skype (stable) $ Last metadata expiration check: 0:00:00 ago on Tue 01 May 2018 04:28:04 PM MDT. Error: Problem: nss-pem-1.0.3-6.fc27.i686 has inferior architecture - nss-pem-1.0.3-6.fc27.x86_64 does not belong to a distupgrade repository - problem with installed package nss-pem-1.0.3-6.fc27.i686
To resolve this, manually install the nss-pem packages from FC28 prior to the upgrade using the following command.
dnf install nss-pem-1.0.3-9.fc28 --releasever=28
After doing so, re-attempt to upgrade and the upgrade should now proceed.