How do I claim for Employment Allowance for a missed year?

Hello ClareQ, thanks for posting on the Community page, If you are using Standard payroll, this can't be done, so you would need to use basic paye tool for that. If you are using Advanced Payroll, you would need to roll back for last year and then turn it on and then roll forward if not then basic tools and the p32. 

How to roll back a payroll- You'd want to unlock the pay run first. Then, delete it. 

Here's how:

View the required pay run in the Pay Runs screen.

Go to the Pay Run Actions.

Select Unlock.

In the dialogue that drops down, enter the reason for unlocking the data and click OK.

Once completed, you can now edit, update and recalculate or delete the pay run. 

Here's how:

Open the unlocked or unfinalised pay period.

Select the red Delete Pay Run button.

In the dialogue that drops down, confirm by clicking the blue delete button.

You refer to this article for more insights about updating multiple pay run periods: Delete a Pay Run in QuickBooks Online Advanced Payroll.

Here is the link for basic tools https://www.gov.uk/basic-paye-tools

If your still struggle i suggest ringing the supportline on 0808 234 5337 and you will be able to set up a screenshare with the agent on the phone and they take talk you though it whilst viewing your screen. The line is open mon to fri 8am to 7pm and is free from any UK landline or mobile.