Costing variant missing

What causes this issue?

If you want to carry out postprocessing on the itemization of the
product cost estimate with unit costing, a costing variant must have
been entered for the unit cost estimate in Customizing for the
valuation variant of the product cost estimate. This entry has not been
made in costing variant &v1&.

System Response

The product cost estimate cannot be postprocessed.

How to fix this error?

In Customizing for costing variant &v1&, enter a costing variant for
the unit cost estimate.

Error message extract from SAP system. Copyright SAP SE.