I can't update my iPhone 6s Plus

Have you tried doing the update to iOS15 by connecting the phone to a computer running iTunes? That is often required when updating older iOS devices. Please see:

Update your iPhone, iPad, or iPod touch with iTunes - Apple Support

Pls contact Apple Care Hotline

Contact Apple for support and service - Apple Support

or download Apple Support App https://apps.apple.com/tr/app/apple-support/id1130498044.

An iPhone 6s, or 6s Plus cannot be updated beyond iOS 15.x.