Well, I now solved the issue. I found a way to query schema of MS Graph with

GET https://graph.microsoft.com/v1.0/$metadata

Then I looked for BookingAppointment and found the expected schema and if it is optional. MS Graph - meta schema

It turned out that some keys are not consistent with what is written in the documentation e.g. end -> endDateTime, start -> startDateTime. I would say MS Graph documentation is very poor and input validation is so poor making it hard to debug.

After correcting the schema, I now successfully create Appointment with the API.