The reason for getting this validation error is because the eBay category that the products it is mapped to does not allow products with variations to be listed within it.

To resolve this, you will need to remap the product to a category that does support variations.

From the product page in eBayLINK, click the Edit button, in the Categories section, click "Add new mapping"

Find a more appropriate category that will have the following symbol,


then click save mapping, which bring you back to the Product page, click update to commit the changes and then go to the dashboard and update market place.

Check back once that has been completed to ensure the product is listed / updated.