Yes, it's usually best to use the latest software version. However, the only non-production server I have access to work with is an older Windows Server 2008 R2. Yes, out-of-date and obsolete on some levels, but for my purposes (intro training only) it's "good enough" at the moment. SQL Server 2019 was throwing errors indicating that it would not install on my 2008 server.
I agree with you that if SSRS can see the server, it is communicating correctly. However, it could not see (or would not list) the SQL Instance. I assumed it was permissions related as I had ruled out the firewall being the issue, but permissions appeared to be correct.
I was able to get around the issue by installing SSRS 2014. It listed the SQL Instance and connected instantly. I would assume that SSRS 2019 would be able to use a 2014 SQL database, so there must be some setting/permission that I'm not aware of.
Regardless, the issue has been resolved. Thanks for your earlier reply.