Head to the Payroll Settings page in the app. Here you'll see a section like the ones in the images below. This will be in one of two states:
If the Employer allowance has not been claimed through Onfolk
If the Employer allowance has been claimed through Onfolk
If you've used a different payroll software or provider between now and the start of the current tax year then you need to double check whether or not you have claimed with them as Onfolk will not show this