Answer
Workflows triggered from a Custom Event or Event Action are restricted to Validation and Process Enrichment types only — a workflow containing a User Interaction step (like a User Task, even a purely informational one with no input required) can't be used as an event-triggered workflow, since event/action-triggered execution happens in a background/system context with no interactive user session for a User Task to actually present itself to. If you need to alert a user without requiring input, that has to be achieved through a different mechanism (like a notification/message sent directly via PL/SQL from the event action, or a Streams-style task assignment) rather than a User Task step inside an event-triggered workflow.