Hello @ClaudioMorales-2195 this might be happening since the user whose calendar you are trying to create the event on has not shared their calendar with the user creating the event. From the outlook-get-shared-events-calendars documentation, an owner can grant a delegate to their primary calendar and act on their behalf, to receive or respond to meeting requests, or create or change items in the primary calendar. To solve this issue, kindly follow this guide on outlook-create-event-in-shared-delegated-calendar which will allow your users to create events on another's calendar.
Let me know if this helps - and if you have further questions,
Diana.