Hello Matt,
The error message you shared looks like it was generated on the server. Displaying server-side messages as validation error is not supported and requires a custom solution. We currently do not have an example matching your criteria, however I would recommend checking
this Code Library project, which shows how similar behavior could be implemented for the Grid widget. I believe the example would help you implement the same for the Scheduler widget as well.
Regards,
Alexander Popov
Telerik
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