Error 21919309: Should contain only one value. Remove the extra values and try again.

This error occurs when you are trying to enter multiple values for an item specific name that only supports one value. Many item specifics only support one value, so it is a good idea to use the getItemAspectsForCategory method to verify which item specifics only support a single value (SINGLE is returned) and which ones support multiple values (MULTI is returned). Please revisit your ItemSpecifics.NameValueList container for that item specific name and remove additional value(s).