First of all lets verify if you are admin

Open control panel > User accounts > Manager other account. Your account should be listed as admin. If your account already showing as admin than your user profile as as issue. 

You can try running SA in hidden admin instead 

Open CMD > Run as admin

To activate the Administrator account, type the command net user administrator /active:yes and then press the Enter key.

Now you will have the second account as administrator and you should be able to run SA in it

Fixes done by SA in admin account will be applied to your account as well. 

I appreciate the response, and I gave it a go, but unfortunately SA continues to stubbornly insist that I am not an administrator. I am listed as the only user on the computer and as an administrator. I'm actively starting to wonder if this program is not available after warranty expiration  or if you don't pay for premium services. I can't think of another way to tell this specific program that I am administrator when no other program on the computer has an issue.

Supportassist can still be used on Out of warranty machine 

Just to confirm that it's not an issue with your user account.

Can you try the steps above and enable hidden admin and run SA in it

I'm sorry I wasn't clear, yes I tried the steps above and unfortunately it didn't work.

Oh I missed that 

I am pretty sure a fix would be released in a few days as there are more users on this forum reporting the same issue

I have this problem. The "fix" is apparently not in.

I am also having this issue, and I tried first with the hidden Administrator account (following the Command Prompt steps) and then with my regular account (which I confirmed while in Administrator that was also set as an Admin.)  I was able to download drivers fine, but it won't let me run the Diagnostic Quick Test.