Discrepancy in underpaid tax- how to obtain PAYE coding notice to check

Hello, I am in the process of completing my self assessment April 2022-2023. I have nearly finished. One section states underpaid tax for earlier years included in the tax code for April 2022-2023 as 'X' amount. I assume this is from underpaid tax from the tax year April 2020-2021. How do I verify this? I cannot find my PAYE coding notice. I have looked at my self assessment calculation from April 2020-2021 which shows I owed slightly lower amount than the X amount. Can I put this value down or is there a way to see my PAYE coding notice to help understand X amount. Or have I misunderstood this? Thank you for your help with this.

Hi a1designs,
Thanks for getting in touch!
You’d normally be able to see exactly how much previous years’ tax you paid through your 22/23 code by checking your 22/23 coding notices.
If you can’t find your coding notices, though, just get in touch with us by webchat or phone, and we can confirm that information for you.
Income Tax: general enquiries
Telephone: 0300 200 3300
Thank you.