QB Issue Resolution:

If you’re filing the return from January to march, you can use the old way of filing. You can then use the new way moving forward.
Here’s how to file the old way:

  1. In QuickBooks, Go to File VAT. 
  2. Select File VAT instead of File VAT MTD. 
  3. By pass the HMRC warning and click Continue. 

Though, if you’re still unable to file the return, I’d recommend reaching out to our support team through these steps:

  1. Click Help at the top.
  2. Select Support. 
  3. Go to Additional Resources and click Contact Us. 
  4. Select QuickBooks Desktop. 
  5. Choose any topics
  6. Scroll down and click Get Phone Number. 

Resolution for Issue 'Quickbooks Pro 2019. VAT MTA Filing failing with Please check the VAT Registration Number and try login again. Done that 4 times still not working. Suggestions welcome.' available: Yes (Solved).
Source: Intuit Community forum.

Quickbooks Pro 2019. VAT MTA Filing failing with Please check the VAT Registration Number and try login again. Done that 4 times still not working. Suggestions welcome.: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

Support for Additional Resources
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