You don't give many details, but it looks like your passport is not stamped and they need other evidence.
Have you ever paid council tax? If yes, council tax statement should do.
Alternatively, if you have ever worked or been self employed, request a statement of taxes an NI contributions from
https://www.tax.service.gov.uk/shortfor ... DPU_SAR_NI
Finally, if you are a student, you may send an academic record for the relevant period.
Otherwise email HO explaining the documents you are able to supply and they will tell you whether they are acceptable.