Hello there, @j-middleton511. I've got steps to help you resolve the error you're experiencing when trying to submit your VAT return in QuickBooks Online (QBO).
The error "We're having trouble connecting to HMRC" usually appears because of HMRC's 18-month reauthorization requirement. It could be that your login credentials used to connect QuickBooks to HMRC are not authorized or have expired to submit a VAT return.
Here's how to reauthorize the connection:
1. Go to All apps, then click VAT.
2. Hit Overview and Edit VAT.

3. Select Edit settings, then turn off MTD.
4. Reconnect QuickBooks to HMRC using the correct MTD credentials.

You can always come back here in the Community if you have other questions or concerns. We're always here to help.