This means this SKU is showing something different in Seller Central, such as different ASIN, Title, etc so they need to verify the MSKU in their Seller Central Inventory to see what it is tied to since Amazon is advising it is not the same Title or ASIN or Both that is showing in their Seller Central inventory.
One example is in case https://inventorylab.zendesk.com/agent/tickets/23421 where she was reusing MSKUs for active listings but for different ASINs. She received the errors but doesn't appear to have corrected them as the list price changed for the listing but nothing else, which caused her to lose a lot of money on the sale of the item.