Hey Lancefromduncan,
Is this an iMac introduced in late 2015 or later or an iMac Pro? Those would meet the requirements for Monterey.
With that said, have you tried this in another user on the Mac yet? If not, we'd recommend creating an Administrator user account, logging in to that, and test updating there: Set up users, guests, and groups on Mac - Apple Support
Add a user
1. On your Mac, choose Apple menu > System Preferences, then click Users & Groups .
If the lock at the bottom left is locked , click it to unlock the preference pane.
2. Click the Add button below the list of users.
3. Click the New Account pop-up menu, then choose a type of user.
Administrator: An administrator can add and manage other users, install apps, and change settings. The new user you create when you first set up your Mac is an administrator. Your Mac can have multiple administrators. You can create new ones, and convert standard users to administrators. Don’t set up automatic login for an administrator. If you do, someone could simply restart your Mac and gain access with administrator privileges. To keep your Mac secure, don’t share administrator names and passwords.
4. Enter a full name for the new user. An account name is generated automatically. To use a different account name, enter it now—you can’t change it later.
5. Enter a password for the user, then enter it again to verify. Enter a password hint to help the user remember their password.
6. Click Create User.
7. Depending on the type of user you create, you can also do any of the following:
For an administrator, select “Allow user to administer this computer.”
For an administrator, select “Allow user to reset a password using Apple ID.”
Let us know how it goes and we'll take it from there.
Cheers.