Invalid Argument Value warning is issued in the following situations:
Unable to validate if the ‘WebReferenceName’ argument of the action ‘<actionName>’ refers a web service consumed in your module because the name supplied is not a literal.
Some actions have parameters that receive the name of Web Services consumed inside the module. The web service name you supplied is not a literal so Service Studio has no way of validating at design time if the consumed web service exists in your module.
Double-click on the error line to take you directly to the argument of the action where the violation occurs.