iCloud email alias' disappeared from my iMac with Monterey and there doesn't seem to be a way to add them back

Hello community!

My iCloud email alias' disappeared from my Late 2015 iMac with Monterey (version 12.7.5), and there doesn't seem to be a way to add them back. I can only use my original iCloud email address on this machine. I can select and use them on every other one of my devices. I first noticed this issue today. An

I've followed the steps on this page, but I still haven't found a resolution: Add and manage email aliases for iCloud Mail on iCloud.com - Apple Support. I don't have the option to add an alias, only an account. See pictures below. When I try to add an alias in this account screen, it does not work because its not an account. What am I missing? Thanks in advance for your thoughts and help!

v/r

Chris