Single object assignment: enter a plant

What causes this issue?

You are attempting to assign a single object to
<DS:GLOS.cost_object_ID>cost object ID</> &v1& for which a plant must
be specified.
However, you did not specify a plant.

System Response

If you don't specify a plant, the system will attempt to use the plant
entered in the master record of the cost object ID.

How to fix this error?

Assign the cost object ID to a plant in the master record, or enter a
plant for the single object.

Error message extract from SAP system. Copyright SAP SE.