Example:

There are a number of reasons why this can happen:

1. This will happen if your product is out of stock. If you just updated your inventory on Shopify, ask them to wait up to 2 hours (though it shouldn't take this long) to see if it updates.
2. Check two areas of your Seller Central account to verify that Amazon is not waiting on any information:

a. The small flag to the right of the Seller Central logo for notifications

b. Select "Performance" > "Performance notifications" to review these notifications

Sometimes this also occurs because a merchant has placed their account in "Vacation mode" (ie. they can't fulfil over vacation mode, so the listing becomes unavailable for purchase). If this is the case, merchants can remove vacation mode from Seller Central.

If none of the above is the reason, it's possible that this account is in "Staging" mode. Merchants need to contact Seller Support to move this from Staging to Production so that they can begin selling!