data agent can't get data from ontology data source

Your understanding is correct that an ontology doesn't have tables in the traditional sense, and the agent's response referencing "select tables in Schema Catalog" is misleading. As per Microsoft's own ontology + data agent tutorial, once you add the ontology as a source, the entity types should appear directly in the Explorer pane with no table-selection step required as the agent is meant to reason in terms of entity types and relationships, not tables.

A few things worth checking:

1. Confirm the ontology graph, and not just the entity, is added as the data source. In your screenshot, the Explorer shows both Ontology_2 (with interview_message as an entity) and a separate Ontology_2_graph_... node. Make sure the data agent's data source is pointing at the ontology item itself, and that the entity type binding is actually published/active, not just visible on the Instances page.
2. Add the known-issue workaround instruction. Microsoft's docs flag a known issue affecting aggregation/query grounding on ontology sources, and the documented fix is to go to Agent instructions and add: "Support group by in GQL". This has resolved similar "can't query, no tables selected" responses for others, even outside pure aggregation questions, because it nudges the agent's query planner to actually use GQL against the ontology instead of falling back to SQL/table logic.
3. On the "Test data agent" hang/timeout — this is a known preview limitation. Microsoft's guidance is: if you get a "no data" or timeout error on first queries, wait a few minutes for the agent to finish initializing against the ontology binding, then retry the same query. It's not usually a config problem if it eventually works.