Error types:
“The account you are connecting to is not a Professional Amazon seller account.” Merchant needs to have this account and not an Individual account in order to use Shopify Marketplace Connect (Amazon doesn’t provide API access for Individual accounts).
Amazon requires additional information to complete its account setup. For example, they may need to provide proof of business address or a passport in order to complete registration. This is not uncommon for merchants who are very new to Amazon. See Performance Notifications in Amazon.
Seller Account might still be in Staging mode (when you first start a store, it can default to this mode). They need to go in and adjust vacation settings to take it out of staging mode or reach out to Seller Support to remove their account from staging mode.
During account setup, Amazon flagged their shop for additional review. For some reason, they failed a “health check” and Amazon is quietly reviewing their account (in this case merchants simply have to wait for Amazon to complete their review).
Amazon has blocked the account, terminated or the account is not registered in the region the seller is trying to sell on. According to Amazon, the next best step is to let the merchant know there is an error with their Amazon account and direct them to contact Seller Support. The merchant may have a notice or alert in their Amazon Seller Central account, or an email with a request from Amazon that may have more information.
Couldn't connect, contact Amazon. As the banner suggests, the merchant should contact Amazon Support: https://sellercentral.amazon.com/gp/mws/contactus.html
The account connection timed out. When a merchant goes through the process of connecting to their Amazon account, they have 15 minutes to complete the process, or else the token will expire. Ask the merchant to start the process over again and complete the setup within 15 minutes.
“An error has occurred”. This error means the issue is on our end and merchants should contact Marketplace Connect support. Please ensure the merchant has tried to reconnect, cleared their cache/cookies, and tried connecting to a different browser. If all of this fails and the error banner persists, please escalate to Shopify Marketplace Connect support. Do not escalate without the merchant's Amazon merchant ID and the time (with time zone) of their more recent connection attempt. Shopify Marketplace Connect support will need this information to check the logs.
For further assistance, please reach out to marketplace-connect@shopify.com