“Unfortunately Samsung Galaxy has stopped” is a common error on the Samsung Galaxy S6. The main reason why this happens is because there is a software error that needs to be fixed.
‘Unfortunately Samsung Galaxy has stopped’ can generally be fixed with a Galaxy S6 factory reset . Most of the time this method will solve the problem, but in doing so you will also lose any not-backed-up data and your customisation options. Another suggestion if the above method did not work is to clear the cache of the Galaxy S6, you can read the full guide here: How to clear the cache on Galaxy S6 
Another issue that is facing Galaxy S6 and Galaxy S6 Edge owners is that the “Unfortunately Samsung Galaxy has stopped” message sometimes relates to a particular app and not the smartphone. Instead of uninstalling and reinstalling an app or clearing its cache will usually solve the issue of apps crashing without you performing a factory reset.
How to Fix Galaxy S6 “Unfortunately Samsung Galaxy Has Stopped” Error:
- Turn on the Galaxy S6
- Go to settings, and then application manager
- Swipe left to go to all the apps
- Browse until you find “Samsung Galaxy”
- Clear cache, then data
- Reset the Galaxy S6