Check if the given Host Name / IP Address is correct.
Run the below command in Applications Manger installed server to ensure Host / IP Address is reachable.
ping <Host Name/IP Address>
Eg: ping apm-server
Check if the given Port is correct.
Retry with the "SSL is enabled" option set to both enabled and disabled states in the add/edit monitor page.
Run the below command in Applications Manager installed server to ensure service is running.
telnet <HostName> <Port>
Eg: telnet apm-server 80