The reason that you are getting this message returned by HMRC is because you have already filed a return for this VAT number for this period.

You can check the submissions made by logging into your online client portal HERE and selecting submissions. You can reprint copies of the receipts from here. If there is a query, please email the VAT number and the period you are trying to file to support@absolutetax.co.uk and the team can check the submission logs.