I got the same error and it took me forever to figure it out but this is what I did and it worked for my store:
Then I went back to Shopify and clicked Deactivate Amazon Pay then Activate which took me back to the Amazon Pay setup. Although it said my account was completely set up, I clicked Next until it came to the Transfer Credentials button. I clicked the Transfer Credentials button and after a couple of seconds it took me back to Shopify Provider Payments.
I retested the Amazon Pay button and it worked.
I hope this works for you.