iMac 2017 Computer won't update to latest OS version(s)

OMGosh, yes! We have Bitdefender installed on all devices. I never thought of this? It's the small stuff one tends to forget (or sweat). Thank you very much! I'll check it out and let the hubby know. This makes a big difference. Does one REALLY need any type of malware, junk-ware or virus protection for an iMac? Thank you SO much for assisting ~ I'm very grateful.

Well, nothing occurred in Safe boot. So I did the reinstall of the MAC OS, which was Big Sur (same version which was on there before all this started). I disconnected all junk-ware. There was an update available for Monterey. So I clicked on it and it didn't upload - don't know why, because I couldn't stay and watch it (darn). So, I tried again to see if the update was available and all that occurred was the spinning beach ball. I found out that the trade-in value on my iMac (2017 21.5" screen Intel 5) is only $150.00 - this sucks. So I don't know what to do now.

Thanks, so much. We'll give this all a whirl.