Self assessment erroneously reports overpaying

I am fully employed but fall into the third category of those who need to fill in a tax return--ie. I am neither self-employed nor a partner in a business partnership. As I am fully employed, all of my tax payments are paid via PAYE, and hence I ought to owe no tax on top of what was paid directly by my employer. My problem is that the self-assessment calculator seems to believe I owe money, but I don't think this should be the case. The figures I used are coming from my P60. Am I filling the form out incorrectly or are one / multiple of my assumptions above mistaken? (eg. am I accounting for something like student loan / National Insurance / pension payments incorrectly?) I can't reach anyone via the phone helplines to address this. Many thanks

Hi materialhazard,

When completing the return, please ensure that you are including the income and tax deducted as shown on your P60.

Also include any allowances there were included in your tax code.

You may have underpaid through PAYE.

Once you have submitted your return, allow 72 hours for processing.

You can then contact our Self Assessment  helpline so we can view the calculation and advise of where the underpayment is from.

Self Assessment: general enquiries

Thank you.