Preceding associations could not be separated

What causes this issue?

Preceding associations should be separated.

System Response

The association between &V1& &V2& and &V3& &V4& contains a quantity
variance in one of the association partners. The quantity variance
affects the total quantity for the other association partner. It is
therefore not possible to separate the preceding associations.
Processing is abandoned.

How to fix this error?

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

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.