There are two potential causes for this error:
- You have not entered a valid Salesforce ID for the &Id parameter in your Composer button URL. This can occur if you hard-code an incorrect ID or an invalidly constructed ID (perhaps using a QVar query)
- You have created a custom field on the Master Object that is also named ID. Even though the Web API name will have a suffix identifying it as a custom field (__c), this can still cause the error listed above as Conga Composer is "confused" by the presence of two ID fields.
WHAT TO DO
Rename your custom ID field (e.g. "Custom ID"), updating both the field label and API field name, and re-run your Composer solution.
You may need to restart your internet browser in order to clear the cache and have Composer recognize the new name of your custom ID field.