Sounds like in-app purchases aren't working on your device. If you try to buy coins do you see the prices list? Please file a bug at Help -> Contact -> Report Bug after seeing the blank button.
As a work-around you can purchase it on the website at
https://www.warzone.com/Idle/PurchaseAutoArtifactSwapper and it will still apply to your device, (just make sure you're signed into the same account)