Why can’t I download apps from my iPhone 6s?

The most common reason is that you're using a payment method not accepted or you have a pending billing issue.

See this support guide:--> If your payment method is declined in the App Store or iTunes Store - Apple Support

See also this support page:--> Subscriptions and Purchases – Official Apple Support

Regards

Giulio

Based on the amount of information you have provided so far your question is unanswerable. When you go to the App Store and try to download an app what happens? What is displayed on the screen?