If you have already filled under Making Tax Digital and you are receiving a 403 error (Client or Agent not authorised), there are multiple reasons this error can be returned but HMRC's investigations have shown that in over 90% of cases the reason is one of the following:
You need to make sure your entering in the correct credentials, either your Agent services account or your client's own filling credentials for MTD.
If you are a private/individual customer you may find What to do when receiving error 403 (Client or Agent not Authorised) - Individual user more appropriate.
Please can you do the following by going to:
After this has been completed, please do the following:
Please note: If you have already fetched the dates from HMRC and you are getting the error 403 when filling online, you need to please delete the period and then re fetch the dates from HMRC and re-map your Excel figures to the relevant boxes.
For more information, please see below attached knowledge based article.