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 are already at 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 you are still on the List & Prep screen, you will not be able to add this item to your batch.
If you are beyond the List stage, the item must be removed from your batch to finish creating your shipment. You will need to close and restore the batch first, then remove the items out of the batch by clicking the three horizontal lines next to the product and choosing 'Delete'. You will then continue with your batch.