Trying to pair a Apple Iphone 6s with an Watch SE

Is this is a brand new Apple Watch SE? If it is, it won't pair with an iPhone as old as iPhone 6s. All brand new Apple Watches come with WatchOS 9. For a watch with WatchOS 9 to pair with iPhone, the phone must be able to run iOS 16 or higher. iPhone 6s is too old to be updated to iOS 16.

If this is your scenario, either return the watch or buy a newer, more modern iPhone.

You can't expect devices to be able to update forever. Your iPhone lacks the hardware required to run the newer iOS versions. The fact you are still using an iPhone from 2015 is remarkable. But it is now over 7 years old and technology moves forward. It always will move forward.