I have MacOS Catalina version 10.15.7, can I have an older version of forScore?

You can have any app you want as long as you get it in the usual and customary ways.

If you're asking will forScore run under Catalina, you need to check their website for system requirements.

If it's available to you in the App Store, then you should be able to run it.

As to the availability of older versions, check with forScore.