You have configured the Communication arrangement as given in 1ZM document to integrate Ariba System.
After manually triggering the replication, you do not see any message in the S/4HANA Cloud Message Dashboard.
After triggering the replication, monitoring of the messages happens via AIF.
For this "Assign Recipients" is required. Once this is completed, you can view the messages in AIF monitoring "Message Dashboard" Fiori app.
In the attached customer guide document S4_Foundation_Master_Data_Integratio.pdf, chapter 5 explains the steps.
Communication Arrangement , KBA , CA-GTF-EI , S/4HANA Extensibility and Integration , How To