Is cancelling a lecture after the attendees have arrived, especially those coming from distant places, considered an injustice that incurs sin?

The trainer's absence from the lecture at the scheduled time constitutes a breach of promise and causes harm to the trainees, as the Prophet (peace be upon him) said: "There should be neither harming nor reciprocating harm." Therefore, cancelling a lecture without a valid excuse involves a kind of injustice, wrongdoing, and negligence, for which the doer will be held accountable.