2019 iMac can't find Monterey
I am trying to upgrade from Catalina to Monterey. Have backed up my system, gone to Preferences then Software update. It does not suggest any, so I have tried "GET" OS Monterey from the App Store. Software update says "Finding Update" and then after the blue line fills up, it quits. Have made many attempts including shutdowns and reboots. My 1 TB HD is only half full. There was no problem updating on my 2015 MacBook pro. Any ideas?
iMac 27″, macOS 10.13