why can't I add a device to my account

Please refer to View and remove your devices that are associated with Apple ID purchases - Apple Support

You may have some older devices that you never removed. Please remember when you are getting rid of a device (sell, trade-in, etc...) to remove it from your account. For example if you have a Mac to remove follow the instructions in What to do before you sell, give away, or trade in your Mac or Apple Watch What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else etc...

You did not answer the question.

How can a device be ADDED?!

There is no question about removing but iTunes and my account do not show an option to ADD a device.

Are both iMacs logged into iCloud or other Apple services using the same AppleID account and info (yours)?

Generally, that's all that is required to assign a device to your account.