Hi there, @rachel44.
The most common reason why HMRC reject EPS submission is due to incorrect credentials for the Government Gateway. You'll want to ensure that you have entered the correct user ID and password when processing.
You can take a look at this article for guidance: Turn on RTI reporting to HMRC in QuickBooks Online Standard Payroll.
See also these resources which you may find helpful:
You're welcome to get back to me if you need further assistance while working in QuickBooks. I’m here ready to help and make sure you’re taken care of. Have a wonderful day.
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