Why can't I download the YouTube app on my iPhone 6s?

YouTube requires iOS 14 and your phone can be updated to iOS 15. Are you running iOS 15? If not, update your phone and then you can download YouTube.

iPhone 6s is supported on and can be updated to iOS 15.

Here is how to: Update your iPhone or iPad - Apple Support

iPhone 6 and earlier are not supported by the current Google YouTube app, so you’ll be using the app pending a change by Google, or using a newer (used, refurb) or new iPhone.

iPhone 8 or newer is required for the current iOS 16.

You can also access YouTube through your browser.