“Sorry there was a problem completing your transaction”

I am trying to log in/start using the app, but when I put in my info (using an iPhone), including my payment, it says: “Sorry, there was a problem completing your transaction. Please try again”. This prevented me from using the app last year, and when I am trying again now I get the same thing. I purchase other things with my iPhone, so I’m not sure what the problem is. Ultimately, this blocks me from using the app.