More than one document for BOM header &1; association with material &2

What causes this issue?

STEP can assign any number of documents to the BOM header of a material
BOM. This is not possible in the SAP system.

System Response

The second and all other documents are assigned to the material.

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.