Fixes for apple pay cash temporarily unavailable.
I ran into the temporarily service unavailable issue for the apple pay/cash options. both to set up and also to send money via apple pay over imessages.
These are the steps i took to fix it.
1. turn off apple pay cash on all the devices iphone/ipad/watch and turn it back on. (Settings->wallet and Apple Pay)
2. check the Language and Region ( Settings->General -> Language & Region)
3. Turn off two factor authentication in the Account settings from Mac or the Account info from iPhone, and turn it back on.
These settings check and resets should help you set up Apple Pay.
Once apple pay is set up you might still not be able to send money over iMessages. for this you might want to ask the receiver to request you for the money instead of you starting the initiation to pay. ( it sounds weird but thats how i sent money over apple pay.)
iPhone 6, iOS 11.2.5, apple pay