FBA guides sellers to align their inventory supply with buyer demand and they check to see if there are sufficient inventory capacities for the given items.
You may have already reached the maximum inventory allowed for this product. This could be due to capacity or other restrictions, including if you removed your inventory during a Free Removal promotion or right before a Long-Term storage fee has been assessed.
If you feel this is in error, please reach out to Seller Support to see if they are able to lift this restriction and allow you to list the item.
If the error persists, the item will either not be allowed to be added to a batch or must be removed in order to finish creating your shipment(s).
If you're working on a Private batch and have already created some of the proposed shipments, you will need to manually close and restore the batch in order to remove the item by clicking the three horizontal lines next to it and selecting "Delete" from the menu.
If you haven't created any of the shipments yet, you can choose the "Decline Shipments & Go Back" button at the bottom of the page to return back to the List & Prep screen
Once there, follow the steps above to delete the item before attempting to create shipments again.