An optional schema step was only assigned values for part of the period

What causes this issue?

Operand &v1& is used as an input operand for an optional schema step.
However, the operand was assigned an operand value for only part of the
billing period.

System Response

An error has occurred because execution of an optional schema step is
not defined for operands that are only assigned for part of a period.

How to fix this error?

Check that operand &v1& of the schema step for the billing period &v2&
to &v3& is assigned completely or not at all. (Note: for additional
information, refer to the details to this message.)

Error message extract from SAP system. Copyright SAP SE.