Error in target system

What causes this issue?

An error occurred when attempting to execute a function module in
another SAP system. The target system may not be active.

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?

Check whether the target system is active. If it is, call version
management again.

Error message extract from SAP system. Copyright SAP SE.