All parameters in RFC require a reference field or reference type

What causes this issue?

You have defined a parameter without a reference field or a type.in a
remotely called function module. This is not supported.

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?

Assign either a reference field or a reference structure to every
parameter.

Error message extract from SAP system. Copyright SAP SE.