I installed kRPC and Infernal Robots (from here) on a fresh install of KSP 1.4.3. I downloaded the Python client (on Python 3.5) using pip, and made this simple script:

import krpc

conn = krpc.connect(
	name = 'foo',
	address = '192.168.1.4',
	rpc_port = 50000,
	stream_port = 50001,
	)

vessel = conn.space_center.active_vessel

group = conn.infernal_robotics.servo_group_with_name(vessel, 'test_group')

When I run it, it fails with this error:

RuntimeError: Infernal Robotics is not available

This is the printed server stack trace:

Server stack trace:
at KRPC.InfernalRobotics.InfernalRobotics.CheckAPI () [0x00000] in :0
at KRPC.InfernalRobotics.InfernalRobotics.ServoGroupWithName (KRPC.SpaceCenter.Services.Vessel vessel, System.String name) [0x00000] in :0
at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (object,object[],System.Exception&)
at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00000] in :0