The HashiCorp release engineering team has recently purged a set of unsupported distribution versions that were previously present.
The Official Packaging Guide seems up-to-date with what’s supported, as far as I can tell. I assume that 7Server was previously included in the repository – probably as a copy of 7, since the Terraform package contents are identical across all distributions anyway – but it has now been removed.
The hashicorp.repo file doesn’t encode any specific release version, but it does include the $releasever substitution, so indeed it will work only if your system replaces $releasever with one of the supported distribution versions. For systems that use unsupported names for that variable, such as 7Server, I suspect you’ll need to manually edit the entry to use just 7 instead to match how the repository is configured.
(I’m just sharing the context I have here. I don’t know anything more than what I’ve mentioned above and am not involved in the maintenance of these repositories.)