Why can't I use my Unlisted Inventory MSKU?

If you are attempting to add an item from Unlisted Inventory to a batch created for the opposite fulfillment channel that it was listed in previously, you will notice that the Use this MSKU button is grayed out.

For example, if you added an item from Unlisted Inventory to an FBA batch, but later attempt to add the same Unlisted Inventory MSKU to a Merchant Fulfilled (MFN) batch, the option to Use This MSKU will be grayed out.  This is not allowed due to the batch being for the opposite fulfillment channel than the MSKU. 

To list the item in a different fulfillment channel, you will need to create a new entry for it in Unlisted Inventory to use.