You'll have to contact the Taxfiler support team. They have the tools to check the settings and configuration of the software why you're unable to connect it with your QuickBooks Online (QBO) account.
Here's the link for reference: https://taxfiler.co.uk/contact-us/.
For more tips in managing your payroll in QBO, visit our Help articles page.
We're always around to help if you have any other concerns. Have a good one.