Problem with a template sensor

Configuration
2

If you’re getting “unavailable” it may be because you are missing defaults on your float and int filters or one of your referenced sensors could be undefined…

Another way to do this would be:

  - platform: template
    sensors:
      nettleie_akt:
        friendly_name: 'Aktuell nettleie'
        value_template: >
          {% set dag = states('sensor.ke_nettleie_dag') | float(0) %}
          {% set kveld = states('sensor.ke_nettleie_kveld_helg') | float(0) %}
          {% set sched = {
          today_at() < now() <= today_at("6:00"): kveld,
          today_at("6:01") < now() <= today_at("22:00"): dag,
          today_at("22:0") < now() <= today_at("23:59:59"): kveld
          }%}
          {{ sched.get(True)}}

FWIW, you are using the legacy template sensor format which will still work, but is no longer the preferred method.