If the costing variant is masked, you must also mask the plant

What causes this issue?

You have masked the entry for the costing variant but not the entry for
the plant.

System Response

Masking ensures that <DS:GLOS.standard_cost_estimate>standard cost
estimates</> are saved independently of the plant under a unified
structure in the system. The results of all standard cost estimates in
all plants must therefore be saved using the same
<DS:GLOS.cost_component_structure>cost component structure</>.
The system searches for costing variants for the standard cost estimate
through masked costing variants. A masked entry for the plant must be
assigned to the masked entry found in this way.
This enables cross-plant transfer of existing standard cost estimates
and ensures data structure compatibility
(<DS:GLOS.cost_component_split>cost component splits</>) between
standard cost estimates in different plants.

How to fix this error?

Either mask the entries for both the costing variant and the plant, or
do not mask either entry.

Error message extract from SAP system. Copyright SAP SE.