My iMac (21.5 inch, 2010) is not listed as being an iMac that can be upgraded to Monterey. What can I do?

Sorry, it cannot be upgraded beyond macOS 10.13.6.

See the following from Identify your iMac model - Apple Support

Mac (21.5-inch, Mid 2010)

Model Identifier: iMac11,2

Part Number: MC508xx/A, MC509xx/A

Latest compatible operating system: macOS High Sierra 10.13.6

A 2010 iMac is long overdue to be replaced. I strongly recommend that you buy a new Mac.

It's too old and considered obsolete by Apple.