This error appears when the requestor has been suspended from Zendesk. In order to fix this error, you need to unsuspend the user by following these steps:
Click the Search icon in the top toolbar
Enter the name of the user you want to unsuspend in the search box and click the user's name when it appears. Alternatively, you can open a user's profile from one of their tickets
Click the Ticket options menu in the upper right, then select unsuspend access
