I have tried to file an EPS return after entering backdated CIS but it keeps rejecting

Hi there, @Grecian.

Allow me to share some details about the EPS (Employer Payment Submission). There are some reasons why you're not able to file EPS in QuickBooks Online:

• You haven't paid SMP or SPP before sending EPS.
• You have outstanding payrolls for the month. Then the EPS filing is listed, but cannot be submitted until you’ve run all payroll for the tax month.
• If you have paid employees within the current pay period (no payment period is indicated), FPS filings have been submitted.

You'll want to make sure that the list stated above must have complied so you can file your EPS to HMRC.

If you're using the standard payroll, you may need to delete any rejected EPS showing on the Payroll Tax screen before you're able to submit the new EPS.

I can show you the steps on how to submit your EPS to HMRC.

1. Select Taxes, then select Payroll Tax.
2. In the Payroll Tax Center, you'll see that if you have an EPS filing that needs to be sent, it is listed under Due Now with a status of Overdue and the date that it was supposed to be sent.
3. Click the EPS and view the details.
4. Select Submit.

I've added this article that  may give you more information about filing your taxes: Submit EPS to HMRC in QuickBooks Online Standard Payroll.

You can also check out this reference if you want to view the submitted EPS and FPS filings: Where to see your filings?

Please don't hesitate to tag my name in the comment section below if you need further assistance or questions. Take care always.