Why I cannot update my iPhone 6s.?

What is the existing version of iOS?

If it is iOS 12.5.4, it is not an iPhone 6S, and can not be updated further.

That would indicate that you have an iPhone 5S, 6, or 6 Plus. Neither can be updated further.

I’m sorry but that’s not true. I have had a iPhone 6plus from 2016 and it is updated to 14.6 and can still be updated to the new one 14.7.1

Yes, you have an iPhone 6S Plus, the original poster is mistaken and does not have an iPhone 6S plus.