An error occurred when connecting to system &1

What causes this issue?

A connection to system could not be made. Possible causes are:
The system is not available
The RFC destination is incorrectly defined.

System Response

The system could not connect to the system. The request can not be
executed.

How to fix this error?

Check the RFC destination, that is defined for the system.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.