The fix for the issue will be included in our upcoming bi-weekly patch release expected to be rolled out in the mid of October, 2019.
NRNevitha Ravi Syncfusion Team October 14, 2019 04:27 AM
Thanks for being patience.
The reported issue has been fixed, now with the latest version 17.3.17-beta you can create appointments when the type of Id is GUID. Kindly upgrade your version to avail the fix and let us know if you need any further assistance.