You receive the following full error message:
Error Creating defect/story in Rally with the following error: Validation error: Defect/Story.FieldName should not be null
This means there is a required field in Rally that is not being sent by Rally Support Manger when creating the defect and that you will need to update your fields mappings in the Rally Support Manager setup. This also occurs if the FieldName field has been set as a required field in Rally. Make sure fields that are required in Rally are populated from Salesforce fields or a default value in the RSM Setup Rally Defect Fields screen.