Synchronization report is running for partner system &1

What causes this issue?

Status message: a synchronization report is already running for
system &SYST-MSGV1&. If a second is started now, it will be
terminated.

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

System Response

How to fix this error?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.