Hello Pablo Veliz,
Thank you for reaching out.
From your comments it seems like when you call https://graph.microsoft.com/v1.0/solutions/bookingBusinesses
It shows the bookingBusinesses Id ( 'XXXXXXXX%40v5dw5.onmicrosoft.com') that you are trying to pull up the appointments for. However, when you call GET /solutions/bookingBusinesses/XXXXXXXX%40v5dw5.onmicrosoft.com/appointments, it doesn't give you any value even though some bookings are scheduled.
I was able to display all the bookingBusiness ID with;-
https://graph.microsoft.com/v1.0/solutions/bookingBusinesses
Furthermore, I grabbed the id and do a GET call https://graph.microsoft.com/v1.0/solutions/bookingBusinesses/bookingtest01@XXXX.com/appointments
It shows the list of List appointments I have.
Please double check if you are entering the correct bookingBusiness ID in the GET /solutions/bookingBusinesses/{id}/appointments call.
Check for multiple bookingBusiness IDs, if all shows no value in the results.
If yes, in that case I would recommend you open a support ticket, there could be a bug causing the issue.
Hope that helps.
Thanks