This article explains that MX doesn't support "set services ip-monitoring" 

Customer may try to run the command "set services ip-monitoring policy Server-Tracking match rpm-probe Probe-Server" on MX104 router and find that it is not available.

We don't find the IP-Monitoring as a valid completion as shown below:

labroot@jtac-mx204-r2045# set services ip-monitoring?  
No valid completions

labroot@jtac-mx204-r2045# set services ip   
                                         ^
'ip' is ambiguous.
Possible completions:
> ip-reassembly        Configure ip-reassembly services
> ipsec-vpn            Configure IPSec VPN service

IP monitoring is not supported on MX that is we do not get options to configure it as it will only work on SRX.

After reviewing the Juniper Documentation and based on the platform we use (MX), the feature IP-Monitoring is not supported.

Please check the following information: Configuring IP Monitoring | Junos OS | Juniper Networks

This feature monitors IP on standalone SRX Series devices or a chassis cluster redundant Ethernet (reth) interface.