i have installed the latest version of iTunes and I am trying to update my iPhone with the latest version of iOS. i keep getting an error message saying i need to update to the latest version of iTunes which i have already done.

It sounds like you are still running Windows 7 or 8, in which case iTunes cannot be updated beyond 12.10.11.2 because the latest versions actually require Windows 10 or 11 - and latest versions are needed for iOS 15 support.

i'm on W10 20H2 and getting the same error

If you are running Windows 10, then you should be able to update iTunes to version 12.12.2.2, which is the actual “latest version” and should solve your error.