This error can be encountered when the user you are attempting to delete from the API Administrative Accounts is in Removed status. A key indicator is if the User Name listed includes a string of numbers with an underscore and the word "removed" following it. Additionally you can confirm if the record is in Removed status. In order to be able to successfully delete an API Administrative Account, the record will need to be reactivated. If the removed record is an Admin record, Contact Support.