Material &2 has no defined relationship to material &1

What causes this issue?

In item &V4& of purchasing document &V3&, material &V1& should be
replaced by material &V2&. However, there is no relation defined from
material &V2& to material &V1&.

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?

See <DS:TRAN.PIC01>Maintain parts interchange relations</> for
maintaining interchangeable parts.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.