Error message: Selected category is not enabled for variation listings.
Summary: This error occurs when you are trying to list a product group to eBay for a category that does not allow multi-variation listings.
When this happens you have a few options.
Change the category for this product group to a category that does allow variation listings on eBay. Your eBay category is in the Category & Description tab when editing a SKU. This is a group attribute, so changing it on the User Interface once changes it for all SKUs in that product group. If you are going to edit this using the import / export feature, make sure to edit the eBay category for the Master SKU. Here's a useful page of multi-variation eBay categories.
Ungroup your SKUs on Zentail so that they are single, stand alone SKUs. This will potentially break up the product group on any other channels or marketplaces this group is listed to through Zentail.