Error 21916724: 

Short Message: Invalid compatibility combination(s). 

Long Message>One or more compatibility combinations are invalid. Name, value, or name-value pair are not recognized.


This is only a warning, which would not prevent you from listing new items or from updating already active listings and eBay would return it, because there are some compatibility combinations in the sent fitment data, that eBay would consider as invalid according to what they have in their catalogue and they would be dropped off. 


Go to Settings -> History, select eBay as Context and place the particular guid in the Identifier field and then hit Apply, so the system could show the related history logs for that item. The invalid compatibility combinations would be shown in the last Log Level 3 report, that have warning in the results column, as they contain the most detailed information, that the channel has returned for that item.