Can you build a json in attribute_template of a sensor?

I’m trying to build a json that is going to be a attribute of a sensor but I get the error

Invalid config for [sensor.template]: expected a dictionary for dictionary value @ data[‘sensors’][‘matsedel_backaskolan_screen’][‘attribute_templates’].

A bit confusing but the loop at the bottom generates the json on the right, if I then feed this to a variable then I can use it just fine.

So when I try and use the same loop in a sensor:

  - platform: template
        friendly_name: "Matsedel Backaskolan skärmanpassad"
        value_template: "{{state_attr('sensor.matsedel_backaskolan', 'weeks')[0]['days'][ (now().timestamp() | timestamp_custom('%w')) | int-1]['items'] }}"
        attribute_templates: >-
                {% for weeks in state_attr('sensor.matsedel_backaskolan', 'weeks') -%}
                  {% if loop.first %}0{% else %}1{% endif %}:{
                  'week': 'Vecka {{ weeks.number }}',
                  {% for days in weeks.days -%}
                     '{{ days["date"]  |timestamp_custom("%a") | replace("Mon", "Mån") | replace("Tue", "Tis") | replace("Wed", "Ons") | replace("Thu", "Tors") | replace("Fri", "Fre") }}dag': {{ days["items"]  }} ,
                    {{"  " }}
                  {%- endfor %}  },
                {%- endfor %} }

I get the error message that it expected a dictionary value.
Is it even possible to do this?