Calendar (local) trigger

Configuration
4 
alias: "TEST: Calendrier Vacances#2"
description: ""
triggers:
  - trigger: calendar
    entity_id: calendar.vacances
    event: start
  - trigger: state
    entity_id: calendar.vacances
    from: "off"
    to: "on"
    enabled: true
conditions: []
actions:
  - action: notify.mobile_app_sm_s928w
    data:
      ... etc ...

Your automation monitors calendar.vacances using a Calendar Trigger and a State Trigger. Why?

I recommend you remove the State Trigger.

The templates in your action refer to various properties of the trigger variable, like message in trigger.to_state.attributes.message, that do not exist when the State Trigger is triggered (or even the Calendar Trigger). That explains why you got an error message when the State Trigger was triggered.