This occurs when the extract program is trying to find a field in a table, other than the lowest table, and the appropriate entry is missing.
For example, a record for the customer number on an order might not exist in the table CUSTOMER, thereby making the order an orphan. The report would then attempt to find the customer details and, when it fails to do so, would report the fact as a join error.
The program prints out the name of the lower table it is trying to join from and the key fields for the table it is unsuccessfully trying to access.
The join error can be corrected by either removing the orphan or adding a parent.