1) Go into system/build.prop and open it in your favorite text editor. Find the line that says ro.config.hw_easywakeup=false and change the false to true then save your changes.
2) Go to system/emui/base/xml and open up the hw_easywakeupmotion_config.xml in your favorite text editor. Find the line that says <EasyWakeupMotion name="Double_Touch" support="1" value="0" flag="0" keycode="131" /> and change the value=0 to value=1 then save your changes.
3) Restart your phone
4) Go into Settings/Smart Assistance/Motion Control and at the bottom you will now see Double Touch. Turn it on.
5) Restart your phone and you will have Double Tap to Wake enabled and working.