adb shell -> su: not found
How can I overcome this problem?
Samsung Galaxy TabS - Android 4.4.2
HTC M8 - Android 5.0.1
The two devices have the same problem.
Are the devices rooted? `su` binary doesn't come with factory shipped devices. You need to root the device (which involves finding and pushing the `su` binary into `/system/bin/` or `/system/xbin/`). See What is rooting?
The procedure I told you is part of rooting. In order to switch to UID 0 (the userId of root user) you need access to `su` and in order to access `su` it has to be present first. How would you move the binary depends upon the device you're using. Some methods are mentioned here: How do I root my Android device?
Your device is not rooted. To see what Rooting is and how to go about doing it:
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.