If you're having trouble accessing the in-game store or browsing the items there, try this classic tech move: shutting it off and on again. Shut down the app and turn your WIFi/Cellular data off and on again before your launch the app again.
For any troubles directly with the purchase, we'd suggest checking the following articles:
For Android players: https://support.google.com/googleplay/answer/1267137
For iOS users: https://support.apple.com/HT203005