Currently, we have an on-going issue where customers, like you, have been unable to file your VAT return. Rest assured that our engineering team has been informed and they're in all hands working for a fix a soon as possible.
That being said, you can contact our Customer Care Support so your account can be added to the list of affected users. This way, you'll receive all the updates once available as they haven't posted any workaround as of this time. To do so:
Go to Help.
Select Contact Us.
Enter Unable to file VAT return in the What can we help you with? field.
Click Let's talk.
Choose a channel where you want to connect with our dedicated support.