Two reasons may cause this error:
1. eBay Category does not support the specified item condition.
Getting this error means that condition of the item was either not specified or not suitable for the category the item belongs to. Some conditions can not be applied in certain categories. So, it is possible that unappropriated condition for the particular category was chosen.
2. Incorrect attribute value.
It is possible to set Condition Value from Custom Attribute. The attribute value must be provided numerically (not in characters) according to eBay Condition Definitions:
a. If you choose Custom Attribute with Text Field Catalog Input Type, make sure numeric value is provided in the certain field when managing the product (in other words, if your item is new, you should enter 1000, but not NEW).
b. If you choose Custom Attribute with Drop-Down Catalog Input Type, make sure that numeric value is provided for the Store View you use for your Listing.
Learn more about item condition for eBay US, eBay UK, eBay Germany, eBay Austria, eBay Spain.