When trying to submit a VAT return under MTD in some cases you may receive an error message: '403 CLIENT_OR_AGENT_NOT_AUTHORISED'.
If you receive the above error message please ensure the following:
When submitting an MTD VAT return you will need to enrol yourself (the agent) to obtain MTD set of credentials. You will also need to ensure that your client is enrolled so the submission can go through. You will not be able to submit on behalf of your client if they are not enrolled for MTD.
For further information on how to enrol your client, please refer to the HMRC link on: Get an HMRC agent services account