This happened to me a couple of times. Apparently it was because I was trying to download a large app (that needs WiFi according to Google) from the PC browser interface. What I did was on my phone, open up the page of the app in Google Play, then stop the download. Then I click again on install. You get a prompt saying that the file is large blah blah may drain your data quota blah blah so you WiFi. I got true unlimited so I just uncheck "use WiFi", then click on "proceed".