You will get this blocking error from Ebay when a mandatory specific required on the ad is not being sent by the module.

Generally, most required specifics are EAN / Brand / MPN. 

If the problem is related to EAN , check the EAN of the product (or its combinations) and make sure it is filled in and valid. If not, make sure you have checked the option: Use “Does not apply” when the EAN is missing or invalid.

If the problem is related to Brand / MPN, make sure you have setup the module to use the product Manufacturer as “Brand” and product Reference as MPN in general module configuration.  
This is the best way to make sure that data will be properly sent to eBay, as long as your website item has a Reference and Manufacturer.

If the problem is related to other mandatory specifications like Model, Type etc.
you will have to configure them from the “Specifics / Condition” tab of the module as explained in the “Specifics/Condition” part of the documentation.