Why can't I get root access from shell? &gt; adb shell
sh-4.1$ su
Permission denied


I have rooted my phone successfully. I know this because I'm able to install apps on SD card and I have a program called SD Maid that is able to operate with root permissions.
From here:

You might need to activate adb root from the developer settings menu. If you run adb root from the cmd line you can get:

root access is disabled by system setting - enable in settings -> development options

root access is disabled by system setting - enable in settings -> development options
Once you activate the root option (ADB only or Apps and ADB) adb will restart and you will be able to use root from the cmd line.