Jason here from Shopify Support.
Your store looks so sleek! Here are the steps we can follow and try to troubleshoot this issue:
1. Log in to your Amazon Seller Central. On the top right corner, click on Settings > Account Information
2. Once inside your account, on the left-hand bottom Your Services panel > Manage
3. In My Services, check both Login with Amazon and Login and Pay with Amazon
Once completed, you can try a test order with Amazon Pay and see if it works. If the issue persists, you'll have to contact Amazon Support to troubleshoot further.
If there’s anything else I can help you out with, please don’t hesitate to reply back and I’d be happy to help out!
All the best,
Here is a great resource if you want to read the bread down step by step guide on Amazon Pay (yours is on point 4 under Activate Amazon Pay).
Let me know how it goes!
So, I just had this problem pop up. Was working just fine then, just stopped out of the blue. A customer brought it to our attention (ugh). Same error message at the OP. What I had to do to fix it was to deactivate Amazon Pay in my Shopify Admin. Delete my applications in my Seller Center Amazon Login account (though, I don't think I had to do this). Then I reactivated Amazon Pay within the Shopify admin and it worked. Hope this helps someone, the help page isn't very useful.
Thank you so much for sharing your valuable tips! It'll help the fellow merchants greatly. I'm sure you aren't alone with this issue (clearly the OP had the same issue before). I'll pass on the feedback to our content team and see if we can offer better steps in the help page. Thanks again!