AHILEN CHAVEZ

I been on the country for almost 3 years working as a graduate student, but I been in the emergency tax code for the same amount of time. I have created my HMRC account and seems my taxes haven't been calculated for the same amount of years. I tried to raise a claim but seems because I don't have a P800 for any of the years so the page doesnt let me continue. Is there is something I can do to get this calculation done, or the tax code comes from the nature of my visa? I will appreciate any advice. Thank you!