Build-in Ad service failed:can not find UnityEngine.Advertisement namespace

When the first time I want to add Ad to my game,I enabled Ad service and UnityEngine.Advertisement namespace works fine.Then later,at some day,when I open my project,there are some errors,they are all caused by one single error:UnityEngine.Advertisement namespace can not be found.I checked my platform,it’s Android.I don’t know what happened,maybe it’s a BUG?
My unity version:5.6.1f1 personal

Problem solved:Switch the paltform to PC then switch it back to Android.