This error means that your agency account is a buyer account, not a contributor account. It is enough to fill out all the required forms and pass an exam for the most of the agencies. If you’re not sure what are you missing to be able to submit - please try uploading a file manually to the agency (over FTP if the agency allows that).
Once you’ve passed all the verifications and the exam on the agency, if you’re sure you have a contributor account and uploading files manually works fine - please try to deactivate the agency in the M+ interface and then fill the login credentials again.
If you’re experiencing this issue with Dreamstime you would need to submit a file manually and wait for it to be approved before your account will become a contributor account and it will be possible to connect it to M+.