Why can’t my phone be upgraded

You need to include details of the version of iPhone that you are using. Some earlier models cannot be updated to the recent versions of iOS. The more information that you can give, the more useful will be the response

Limits of iOS level for various iPhone versions:

5S/6/6plus iOS 12.5.7

5C iOS 10.3.4

4S iOS 9.3.6

4 iOS 7.1.2 

6S/6S Plus  iOS 15.7.3

7/7plus iOS 15.7.3

SE (2016, 1st generation) iOS 15.7.