Problems with apps are occasionally caused by the iPhone's memory running out. Typically, you will see a notification prompting you to free up some space since your iPhone's memory is running low.

Go to Settings > General > Storage & iCloud Usage > Manage Storage to see your iPhone 6s' current memory status.

If required, try checking each program's information to see if there is any typical memory usage on each app Update iOS.

It's a good idea to keep your iPhone's iOS version up to date to guarantee you have all of the required add-ons, features, and bug patches loaded. So, if the issue you're having with the camera app is caused by bugs, the remedy patch included in the most recent version has a better chance of fixing it.

Go to Settings-> General-> About -> Software update to see if there are any available updates for your iPhone.

If an update is available, follow the onscreen instructions to download and install the iOS update.