adb shell -> su: not found How can I overcome this problem?

Devices:


Samsung Galaxy TabS - Android 4.4.2
HTC M8 - Android 5.0.1


The two devices have the same problem.
Your device is not rooted.  To see what Rooting is and how to go about doing it: 


http://www.androidcentral.com/root
What does &quot;to root a phone&quot; mean?


Once you have rooted your device, you will be able to access the Super User state of your device (SU). Just be aware that rooting could brick your device if done incorrectly, as could and tampering with files in the root directory.