Can't use Google Play app from Restricted Guest Account
From this Google Product Forums Post:


  "it is not possible to be able to grant access to the Play Store. You are however
  able to grant access to the different Play apps like Books, Movies and Music. 
  They will have access to the content you have."


So I'm sorry, but this is a product limitation. After all, why would you want a restricted user to be able to install apps? That sounds a bit purpose-defeating.