Can I upgrade from Catalina to Monterey

Yes, if your iMac is a late 2015 or later model. Go to Apple menu > About This Mac for the model year.

See: macOS Monterey is compatible with these computers - Apple Support and How to upgrade to macOS Monterey - Apple Support

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install.  For Best results use Safari to commence the download as Others may not work.

Safari will open a redirect and open the Apple Store where the download can commence

Notation: Rare , but have seen two or three occurrences where moving from Catalina and jumping Big Sur has been problematic. In those specific cases - the move to Big Sur and then to Monterey has solved their issue.

Tips Monterey Upgrade  Released Oct 25, 2021

Make Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal  operation of the OS and may even Corrupt the OS requiring a Reinstallation

If this is the case, you need to update your list of computers that CAN be updated. I have early 2015 Macbook Pro and the list shows that my model should be able to be upgraded. Based on many others though, it sounds like Big Sur may be more stable anyway, so many it is a blessing in disguise.

Point # 1A - There are reports appearing on the ASC Forums regarding failed installation of Monterey on older Qualifying Apple Computers. In most cases this is related to having a NON Apple Original internal drive ?  

The EFI Boot ROM included in Monterey is verifying for Original Apple Drive and if not found , may Balk or fail to upgrade to Monterey. 

Reports indicate the EFI BOOT ROM is Required to Flash the Computer and is a  requirement for a Successful Monterey Installation