Question:

Users may customize a validation rule for Medial Event to prevent none attendees before submitting for approval. However, when clicking the Approve button, the Medical Event Validation Rule is not triggered.

Answer:

The validation rule cannot be used at this point in time as it is a by-design limitation.

As a workaround, the user can proceed with the two options below:

1. Change the Approval Process entry criteria to prevent the approval process from starting if there is no attendee on the field.
2. Create a custom button that invokes Screen Flow which has all the conditions and shows the right error message to the user. If none of the conditions are met, then the Approval process is called from the flow.

Related Documentation:

CRM Help Documentation: Medical Events