There appears to be a typo in https://puppet.com/docs/pe/2019.8/known_issues_pe.html#known_issues_pe
It looks like someone did a search and replace of master for primary server. I think the items highlighted in red should still say master.
When you use Code Manager or r10k to deploy modules from a Git source, the default branch of the source repository is always assumed to be primary server. If the module repository uses a default branch that is not primary server, an error occurs. To work around this issue, specify the default branch with the ref: key in your Puppetfile.