When signing in to Apple ID on iPhone 6 it says "Software update required"

I have an iPhone 6. It is currently on iOS 12.5.7 and it says my iPhone is up to date, obviously since iPhone 6 doesn't support later versions. However, when I try to sign in to my Apple ID it says "Software update required. Update this iPhone to the latest version of iOS to sign in." I don't see why considering it is the latest supported version. I was previously signed in to my Apple ID, however, the other day I signed into my grandma's Apple ID in order to help her with something, and ever since then I haven't been able to sign in. Does anyone know what the issue is, or any possible solutions?