Persistent 404 Error on customerClients:create - All new Test Accounts fail

[Shain]

I am experiencing a persistent "404 Not Found" error when trying to create a new test client account under a test manager account. This issue occurs even on brand new accounts and when using the OAuth Playground, which proves it is not a code issue.

What I've Tried:
Created four different manager accounts; the problem persists on all of them.
Verified I am using a Test Manager Account ID.
Verified I am using a Test Developer Token.
Verified the authenticating user has Admin access to the manager account.
Verified my OAuth Consent Screen has the .../auth/adwords scope and successfully re-authenticated.

Minor:
I was following Gemini Troubleshooting instructions and now have 2 main manager accounts each with a child manager account. I there any way I can delete those.

Thanks