With VPC 7.4.2 I was seeing an error "Repo tag rhel-x86_64-as-4 not defined in the Repo tag file /usr/nsh/patch/rhpu/Scripts/redhatrepo.conf". The problem was that it was defined the file.
While reviewing the code in rhpc.pl, I found that there were a couple of variables referenced around line 1500 that looked a bit suspicious. It's also possible that my use of underscores and dashes tripped up the regular expression used to split the line in redhatrepo.conf.
To make a long story short, there's a simple solution. Use the same name for your OS platform, the directory of your repo, and the name of your tag in redhatrepo.conf.
For example, this line from redhatrepo.conf worked for me:
But this line, which uses a unique identifier for each, did not: