This error occurs when Report Designer is trying to decide how to join two tables together to produce the output and it cannot figure it out. For example, consider a parent report with two child document objects. The parent contains some child data objects and several custom fields that do calculations with these objects. However, if the parent report does not have an actual column (i.e. not a child object), this error will occur. The parent report must have a field on it that comes directly from the metadata file. If you do not wish to see this field in the output, add it to the report and then right-click it and choose Hide Toggle. It will remain hidden but will allow the query to be built without error.