Optional variables and select options may not be used here

What causes this issue?

You want to use a global variable with an optional replacement type
(optional variable) or a select option. However, given that a unique,
single input value is required that should also not be initial, these
variables are not permitted here. For this reason, you should use more
suitable variables.

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?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.