Ref. material for consumption already assigned to material & in plant &

What causes this issue?

You wanted to create or change a material and assign material &V1& in
plant &V2& to it as a reference material for consumption. However, a
reference material for consumption is already assigned to material &V1&
in plant &V2&.

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?

To assign material &V1& in plant &V2& as a reference material for
consumption to the material you are processing, no other reference
material for consumption must be assigned to material &V1& in plant
&V2&.

Error message extract from SAP system. Copyright SAP SE.