Use the new modern template sensor format-
Your new template sensor should look like-
template:
- sensor:
- name: "EndesaOK Utility Meter"
unit_of_measurement: "kWh"
device_class: energy
state_class: measurement
state: >-
{{ state_attr('sensor.endesa_energy', 'energy') /1000*400 }}
Note: Legacy template sensor format does not support measurement state_class.