Maintain either period or period flag

What causes this issue?

In Customizing for Product Costing, costing type &V2& is
assigned to costing variant &V1&. For this costing type, you can either
enter the period yourself or have the system enter it from the material
master using the period flag. You may, however, choose only one of the
two alternatives.

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?

Enter either the period OR the period flag.

Error message extract from SAP system. Copyright SAP SE.