At Universal Containers, a Platform App Builder has built a record-triggered flow on Opportunity that...

The Custom Error element allows a record-triggered flow to prevent the triggering database change and present a targeted error directly to the user. Salesforce supports displaying the message either at the record-page level or as an inline error on a specific field, which exactly meets the requirement to place the error on the Opportunity Stage field. When the Custom Error element executes, Salesforce rolls back the record change until the user corrects the problem. A Screen element is not valid for providing this experience inside a background record-triggered flow. An Assignment on a fault path merely stores values and does not automatically surface a validation-style error. Ending the flow also does not prevent the record from being saved. Therefore, the Custom Error element is the correct declarative equivalent to validation-style feedback. Salesforce

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