It looks like this is a bug. Until it is fixed, you can still add line sensors to LF simulations using the API.
Here is a quick example that adds an entity named 'Lines 1' as a voltage sensor:
# -*- coding: utf-8 -*-
import numpy
import s4l_v1.document as document
import s4l_v1.materials.database as database
import s4l_v1.model as model
import s4l_v1.simulation.emlf as emlf
import s4l_v1.units as units
from s4l_v1 import ReleaseVersion
from s4l_v1 import Unit
# Define the version to use for default values
ReleaseVersion.set_active(ReleaseVersion.version8_0)
# Creating the simulation
simulation = emlf.ElectroQsOhmicSimulation()
# Mapping the components and entities
component__plane_x = simulation.AllComponents["Plane X+"]
component__plane_x_1 = simulation.AllComponents["Plane X-"]
component__background = simulation.AllComponents["Background"]
component__plane_y = simulation.AllComponents["Plane Y+"]
component__plane_y_1 = simulation.AllComponents["Plane Y-"]
component__plane_z = simulation.AllComponents["Plane Z+"]
component__plane_z_1 = simulation.AllComponents["Plane Z-"]
component__overall_field = simulation.AllComponents["Overall Field"]
entity__line = model.AllEntities()["Lines 1"]
# Adding a new VoltageSensorSettings
voltage_sensor_settings = emlf.VoltageSensorSettings()
components = [entity__line]
simulation.Add(voltage_sensor_settings, components)
# Editing AutomaticGridSettings "Automatic
automatic_grid_settings = [x for x in simulation.AllSettings if isinstance(x, emlf.AutomaticGridSettings) and x.Name == "Automatic"][0]
components = [entity__line]
simulation.Add(automatic_grid_settings, components)
# Editing AutomaticVoxelerSettings "Automatic Voxeler Settings
automatic_voxeler_settings = [x for x in simulation.AllSettings if isinstance(x, emlf.AutomaticVoxelerSettings) and x.Name == "Automatic Voxeler Settings"][0]
components = [entity__line]
simulation.Add(automatic_voxeler_settings, components)
# Update the materials with the new frequency parameters
simulation.UpdateAllMaterials()
# Update the grid with the new parameters
simulation.UpdateGrid()
# Add the simulation to the UI
document.AllSimulations.Add( simulation )