Underpaid tax figure - No PAYE Notice of Coding

I'm completing my self-assessment and everything is fine apart from the section titled "Underpaid tax included in PAYE coding (Page 2 of 2)". In this section it states "Our records show that your 2022 to 2023 PAYE Notice of Coding includes an adjustment for tax underpaid." however the figure stated is £0, I don't believe £0 is correct, as I upgraded company cars and was still paying for the less valuable car for most of the year, so I should have underpaid tax. The issue is I don't believe I received a notice of coding this year (possibly due to a house move), so I don't know what the correct figure to insert is. I have looked online, but can't seem to locate the Notice of Coding anywhere on my account. I can view my 2022/23 end of year tax code, which is K99X and states "£990 needs to be added to your pay or pension so the extra tax can be collected." If I can't locate my coding notice anywhere, do I enter 20% of £990 = £198 as the additional tax payable, as my earnings fall comfortably within the basic rate range.

Hi,

You would need to contact our Self Assessment team so we can review the code to check if an underpayment was included.

Self Assessment: general enquiries

Thank you.