can't find Mac OS server 5.1 to download

If by PC you mean non-Apple hardware running Windows then you won't find it and even if you did it would not install. If you're using PC generically to mean any Personal Computer (which would be non-Apple and Apple hardware) then you won't find older versions on the App Store on Catalina as only the newest version is listed. For older versions I think the procedure (having not done this myself - perhaps others will verify?) is to "purchase" the newest one and then look for the older version to download. This would mean you having at least two Macs (perhaps one can be installed virtually on the same Mac) one with the relevant OS for the version of Server you're interested in and the other with Catalina installed. The following article from MacWorld should help:

https://macintoshhowto.com/osx/how-to-get-an-old-version-of-macos-server.html

https://discussions.apple.com/thread/251278474?answerId=252631013022#252631013022