Thank you for posting here in the Community. I'm here to help provide information about the VAT return in QuickBooks.
Once a user marks a tax return as filed, you can no longer undo the tax filing on your end. You may need to contact QuickBooks Support for unfiling.
Before speaking to a support representative, please ready the required information. They'll be needing your QuickBooks Online Company and which tax filing period you wish to unfile.
To location the Company ID:
I'm also linking an article with additional information on unfiling a VAT return:
In case you need the steps on how to contact support, please see the steps I'm adding below:
Please reach out to me here should you need any further assistance with unfiling tax returns. I'm always available to talk QuickBooks.