VAT Return not received by HMRC

Received a letter from HMRC VAT office stating that they have not received a submitted VAT Return. I have a VAT Return confirmation form. On calling HMRC they state that the confirmation form is produced by the submitting software.
They have advised to resend the VAT Return. When I open the VAT Return Form in Datadear and reconnect to HMRC, there is no ‘post’ button.
How do I resend the VAT form ?