Enabe Developer Options on Android and then enable ROOT settings:
To enable Developer Options on an Android phone or Tablet, do the following:
Go to Settings. Click on About phone and then Tap Seven times on the Build number.
Here is a link explaining How to Enable Developer Options on Android or CyanogenMod or any other Custom ROMs.
Now to enable ROOT access, open Developer options and then scroll a little bit down on the Developer Options settings and look for the Root access setting, which must be showing Disabled.
Just click on the Root access and set it to Apps and ADB or Apps only or ADB only, as per your need.
Here are some screenshots for the above settings: