I can't update macOS Catalina 10.15.7
I can't update macOS Catalina 10.15.7 on my iMac (21.5-inch, Late 2013)
iMac 21.5″, macOS 10.15
Question marked as
Catalina 10.15 is the highest macOS that a Late 2013 21.5" iMac can run.
see > Identify your iMac model - Apple Support
Sadly if you need to run a later macOS version, then you need a new or newer Mac.