How to update an old iMac

Which Mac is it? (Model name and hardware mode year / time of year, from About This Mac) That will determine how far you can upgrade macOS, if you can upgrade it at all.

If this is an iMac with a hardware model year of 2011 or earlier, you cannot upgrade it past macOS 10.13.* (High Sierra).

FWIW, you need a Late 2012 or later iMac to update beyond High Sierra 10.13.6