Enter an RFC destination for the system &

What causes this issue?

You can only enter SAP Systems with Release 4.5A or later from the same
transport domain. To choose other SAP Systems, you must enter an RFC
destination.

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 an RFC destination for the required SAP System.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.