Thanks for joining this thread, @Don 333. I can share some information that can help you resolve the error that you cannot connect to HMRC in QuickBooks Online (QBO).

In QBO, here are some of the troubleshooting steps that you can try to resolve the error you're having with connecting to HMRC:

• Check the email confirmation that you've received from HMRC to confirm that your connection has been successful.
• Make sure that the VAT number matches HMRC's record.
• Using only the official MTD credentials provided by HMRC via email. 
• Setting up MTD and authorising QuickBooks to connect to HMRC.
• Updating your VAT accounting period from your Taxes page.

For detailed guidance about fixing a specific error message or having issues when connecting to Making Tax Digital (MTD), please refer to this article: Fix MTD issues and errors in QuickBooks Online.

Moreover, we can perform some troubleshooting steps to determine if this problem is caused by a corrupted cache on your browser. Let's begin with accessing your QBO account using incognito. Private modes do not store cache files and will give you a clean slate to prevent errors when running websites. Here's how:

• Google Chrome: press Ctrl + Shift + N  
• Mozilla Firefox: press Ctrl + Shift + P
• Safari: press Command + Shift + N

If you're able to connect to HMRC using a private browser, go back to your regular browser and clear its cache. Clearing the cache or cookies is the best way to get a clean slate for your browser. Otherwise, using other supported browsers can be a good alternative too.

When everything is fine, check out this helpful link to guide you in submitting your VAT return: Submit a VAT return to HMRC using MTD in QuickBooks Online.

Let me know if you need further help connecting to HMRC in QBO. I'm always glad to help in any way I can. Have a great rest of the day!