Hi Adam.
I tested in my lab (Exchange 2016 CU20) and got the same error message.
According to this link: Clear-TextMessagingAccount
The Clear-TextMessagingAccount cmdlet allows a user to remove the text messaging settings from their own mailbox.
An administrator can't use this cmdlet to remove the text messaging settings from another user's mailbox.
It is by design that admins cannot use this command to change the text messaging settings of other users' mailboxes.
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