I got an email about the Amazon barcode system. I applied it as shown in the email and followed these steps:
In Seller Central, I went to Inventory > Manage All Inventory.
I used the search bar to find the affected SKU by entering its ASIN or SKU.
Next to the impacted ASIN or SKU, I clicked the three-dot menu and selected Add another condition.
On the offer creation page:
I entered a new seller SKU and completed all required fields.
I clicked Submit/Save and Continue.
I then got another email saying I need to change the Amazon barcode system, and that this also applies to my new listing, even though I already gave them a new Amazon barcode system SKU (X00).
It is not always available during listing creation. Go to Manage Inventory, edit the listing, then check the Fulfillment Channel or Offer tab and look for the barcode setting there.
If you still do not see it, it usually means Amazon has already defaulted it based on the category or your account settings. In that case, just go to Send to Amazon, and it will generate the FNSKU labels for you anyway.
If nothing shows up, the worst-case scenario is to open a quick case with Support, as they can switch it on their end.
I found the solution. First, I needed to change the FBA Amazon Product Barcode Preference setting to Amazon Barcode, then create new listings. I hope this helps others who are struggling with the same issue.