QB deducted Employers Allowance from PAYE payments last tax year, but did not report that to HMRC, so HMRC thinks I have underpaid. Can I re-submit anything to correct?

Welcome to the Community, @support48.

You'll have to make sure that payroll data is correct so you can resubmit Employer Payment Summary (EPS) to HMRC. Let me guide you on how to resubmit EPS to HMRC.

1. Ensure you have correct all weekly and monthly payrolls for the month.
2. Click Employees from the left panel, then select Approved and Advanced option.
3. Select Make a different RTI Submission.
4. The second line on the form shows the Employer Payment Summary (EPS) and click Submit to the right of the table.
5. Complete the form as necessary and click Submit EPS.

Here's an article you can read for more details: Employment Allowance.

To more about Employer Payment Summary (EPS), feel free to check out this article for more information: Employer Payment Summary (EPS) in QuickBooks Online Advanced Payroll.

Please know you can continue to reach me here with any additional questions. Thanks for coming to the Community, wishing you continued success.