The selected attribute is not allowed as a variation specific

Depending on the category, eBay restricts certain attributes (e.g. Size, Color, Model, etc.) to be used as product variation specifics. 

To resolve the error and comply with eBay regulations, you would need to remove or change the restricted variation specific. 

If your listings are flagged as having invalid variation specifics in error, reporting the issue to the eBay support team would be the best solution in this case.