Can I run macOS 10.15.6 on a 2017 iMac?

Understand that macOS 10.15 and higher cannot run many older software packages. Older apps like Office 2011 and older and old version of of Adobe Creative Suite will not run if you upgrade. They will need updates on in the case of the wo expensive suites I mention, purcahsing new versions.

If you are in the middle of a big project that requires older software version, I recommend upgrading no higher than macOS 10.14 Mojave until having a software suite stop working does not cost you an assignment or cause a lot of data loss.

You can use this tool to check for Catalina-compatible (64-bit) apps on your computer:

https://www.stclairsoft.com/Go64/

You should be able to run Big Sur on it. If you have a spinning hard disk it may be rather slow: if you have an SSD or a fusion drive you will probably be OK. You can run Catalina if you prefer.