Hazmat reviews are generated directly from Amazon and, unfortunately, we have no control over this besides displaying the error when they send it over. While you may see the item available on Amazon as a Merchant Fulfilled item, the error you received is indicating Amazon is not currently allowing the item to be listed as an FBA item.
Once the hazmat review is lifted, Amazon will send the update through the API and this should resolve the error in InventoryLab.
We suggest opening a case with Seller Support if you have further questions as to why the restriction is being placed on the ASIN.
Not sure if the item you are trying to list is a dangerous good or under hazmat review? Click this link to the Amazon Seller Central Dangerous Goods Identification Guide (Hazmat)