There is no option for me to buy more space on my iphone

Thank you for using Apple Support Communities. I see your asking about iPhone storage? Or iCloud storage? If iPhone storage, you can’t buy more unfortunately but with iCloud storage you can, if you need more iCloud storage then go to settings>Name>iCloud>Manage storage> And then change storage plan

here is a link on the key differences between iPhone storage and iCloud storage https://support.apple.com/en-us/HT206504 i hope i was able to help, please feel free to ask any questions, have a great day