By design adb root command works in development builds only (i.e. eng and userdebug which have ro.debuggable=1 by default). So to enable the adb root command on your otherwise rooted device just add the ro.debuggable=1 line to one of the following files:
/system/build.prop
/system/default.prop
/data/local.prop
If you want adb shell to start as root by default - then add ro.secure=0 as well.
Alternatively you could use modified adbd binary (which does not check for ro.debuggable)
From https://android.googlesource.com/platform/system/core/+/master/adb/daemon/main.cpp
#if defined(ALLOW_ADBD_ROOT)
// The properties that affect `adb root` and `adb unroot` are ro.secure and
// ro.debuggable. In this context the names don't make the expected behavior
// particularly obvious.
//
// ro.debuggable:
// Allowed to become root, but not necessarily the default. Set to 1 on
// eng and userdebug builds.
//
// ro.secure:
// Drop privileges by default. Set to 1 on userdebug and user builds.