Yes, you would need the REST credentials. The error that you’re getting is pretty straight forward, and I’ve never seen that error unless there was indeed a problem of some sort with the credentials.
Are you absolutely sure you have the values entered correctly? If so, you’ll need to submit a ticket to PayPal MTS and see if they can help you because it would be out of my hands at that point.
It would make it so the plugin accepts it, yes, but that doesn’t mean PayPal will accept it. I have not specifically tested any other currencies. These were pulled from the documentation we used to develop from.