Unable to Downgrade after upgrade to Android M from Android L I upgraded my MOTO G 3rd Gen (XT1550) to Android M. After that I tried to root My Phone but unable to Root Android M. So I decided to Downgrade to Android L. But also unable to Downgrade.
Seeing the exchange of comments in other answer and the link provided, the problem is with SuperSu:

Version 2.62 ( from memory) is normal SuperSu

Version 2.76 is system less root, which provides root without modifying system partition. My device is rooted with this and I had the same problem. 

Systemless root requires different treatment.

First thing to do is shut the device by adb shell reboot -p . Next is to boot into recovery, enter TWRP terminal and key ''echo SYSTEMLESS=true&gt;&gt;/data/.supersu'' without quotes ( I had read of this in this XDA post

After this reboot into recovery for custom recovery to replace stock. Else in some current devices the custom recovery is replaced by stock. See this for more Booting into twrp unsuccessful