GNURoot Debian is no longer being maintained and does not work on newer versions of Android. You want to use UserLAnd instead: GNURoot Debian only remains for people with devices that are older than UserLAnd supports (below android 5. 0) and to inform people of UserLAnd.
Download the free setup package of GNURoot Debian and take a look at users' reviews on Droid Informer. The app can be launched on Android devices 2.2 and above. The download of this application is safe and secure. Corbin Champion is the official developer of GNURoot Debian which can be found in the Tools category.
Over 0,5M users set up the app and the latest update was launched on Aug 3, 2018. Users rated this application 4.2 on Google Play. A lot of users set up GNURoot Debian on their Samsung and LG devices.
As users mention, the pluses of the app are a nice interface and innovative features.