The Item is either not active or no longer available in eBay database

This error message indicates that you must list a product before attempting to revise it. 

If the item has not yet been listed on eBay or was previously listed but has since been removed, please execute the List action to publish your listing onto the marketplace.

You may also enable automatic List and Relist actions in the Synchronization Policy assigned to your products. It will automatically trigger the right update for your listings. 

If a listing on eBay has been removed without the knowledge of M2E Pro, the only way to reactivate it on the marketplace is through a manual listing action.