Error:

What does this error message mean?
Invalid item for MSKU (SKU) with reason ITEM_STRANDED.

Resolution:

There can be multiple reasons why an item may become inactive or stranded.

All of this must be done on Seller Central before the item can be listed using SellerChamp.

This error is originating directly from Amazon.

If the listing is still there, just do a minor update (price, title, etc.) to see if it become active again. If not, use the Inventory Loader upload with the SKU, ASIN, and Price. Failing that, create a case with Seller Support and advise what you tried and they can work on it at their end. 

Note: If you have units of an ASIN stranded in the fulfillment center, do not send additional inventory of that ASIN to Amazon.
If you add an ASIN with stranded inventory to your shipment, you will receive an alert and your shipment will not be processed.

Also, when you delete or close a listing, or change it from Fulfilled by Amazon to Fulfilled by Merchant, it may strand your inventory.
Product listing issues, infringement concerns, or system errors can also result in stranded inventory.

If your inventory remains stranded, you will receive email notifications from Amazon. 

Thirty days after this notification, your inventory will be classified as unsaleable.
Once it is classified as unsaleable, you will have 30 days to remove your inventory or it will be disposed.