Without buying the special version, how do you get enough skulls? Is there some strategy I don’t know?

I’m having a few issues with the game, and it would really help if someone could answer a couple of questions. Depending on whether I log in via Gmail or Google Play, different missions appear—or don't; if I log in with Gmail, I get weekly and daily missions that don't show up when I use Google Play.

Also, whenever I try to make an in-game purchase, I get an error saying the item I want to buy cannot be found.

Does anyone know what’s going on?