Why am I unable to Sign In with Apple ID on my iPhone7

I just got a free government iPhone 7, I was able to update it to the latest version of IOS 15.7.3 but I’m still unable to Sign on my Apple ID. I get a little pop-up window saying that I cannot sign into my Apple ID because it can only be signed in to the latest version IOS 16 or later models. That I cannot update this iPhone to the current iOS version. Don’t know how to add my Apple ID to this old phone… I’m so confused???