macOS catalina install error - this disk doesn't use GUID partition scheme (Au Contraire!)
Solution 1:
Catalina will only install to APFS, which probably means you will have to install High Sierra first - which will install on HFS+ & convert as it runs.
See How can I download an older version of OS X/macOS? for deeper info on this in general [which unfortunately has got rather long & confusing over the years.]
Jumping this far… definitely have a known-good backup before you start.