Source container: Deviating no. of lines in correspond. source container

What causes this issue?

According to Customizing, a table of object references should be
created as the target reference. The table elements of the source
container from which the object key should be created, do not have the
same number of lines.
Following target element is affected: '&1'

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Transfer the table elements with the same number of lines.

Error message extract from SAP system. Copyright SAP SE.