When posting config text please use code blocks </>, or this, not the quote option.
So the template integration goes in your configuration.yaml file:
template:
- sensor:
- name: "Nozzle Temperature Current"
unit_of_measurement: "°C"
state: "{{ state_attr('sensor.duet_status', 'temps').current[1] }}"
availability: "{{ state_attr('sensor.duet_status', 'temps').current[1]|is_number }}"