Thank you for reaching out.
Couple of things to check here -
- When you are trying to get users' calendar, you are using user id or userPrincipalName properly in your Get request.
- Make sure you have the necessary permission to call this API. Refer documentation here for this.
- Check your access token on https://jwt.ms and see if your token has all the necessary scopes.
If you are still facing issues, please share your client-request-id and timestamp so that we can look into it.
Thanks.