The operation has been referenced. You cannot change the data

What causes this issue?

The operation for which you want to calculate standard values is
referenced from a reference operation set. You cannot calculate
standard values for referenced operations.

How to fix this error?

To calculate standard values for a referenced operation, proceed as
follows:
Call up the reference operation set for maintenance and calculate the
standard values there or
unlock the referenced operation set.

Error message extract from SAP system. Copyright SAP SE.