A multiple assignment is only possible on a cross-category basis

What causes this issue?

There is an overlap between the validity periods of the entered
assignments. When multiple assignment is activated, no overlaps are
permitted in a category. Overlaps are only permitted when a secondary
assignment is entered in a category alongside the main assignment.

System Response

The system does not transfer the entered assignments.

How to fix this error?

Adjust the periods of the overlapping assignments if you have selected
article hierarchy nodes within the same category, or create the
overlapping assignment as a secondary assignment within another
category.

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.