As per SP the shop is closed temporarily while they investigate and resolve a critical team wars issue:

Hello, Monster Masters!

We have temporarily closed the shop (War Coins) while the team is working as quickly as possible.
to fix the issue we currently have.

I want to let you all know that the Team is currently looking into what has happened in regards to some players taking advantage of this situation.

We apologize for the inconvenience and thank you for your patience while we work on this.