IPhone can’t install the last version

Your post is a little unclear, 15.7 is the highest you can go on a 6S or are you trying to update to that version?

What happens when you try?

Can you post a screenshot of the whole screen, it could reveal why you aren't able to update.

Make sure you:

• Are using a WiFi connection (not a hotspot).

• You have an active cell signal (You need both WiFi and cellular service to perform the upgrade).

• Are not using a VPN connection.

• Have your battery charged at least 50%. Better yet, keep it plugged in during the update.

• Do not have power save mode turned on.

• Do not have a beta version installed.

• You have at least 6 gigs of storage available.

You can also check out this article for help -

If your iPhone, iPad or iPod touch won’t update

If your iPhone or iPad won't update – Apple Support (UK)

I suspect you have a beta version installed, follow these directions.

​ How to uninstall iOS or iPadOS beta software - Apple Support

Your welcome, did you get it installed?

Did you follow the instructions for removing the beta version?