1. Create a custom report type with Primary Object “Chat Transcripts”.
2. Select Chat Transcript Events to relate it with Chat Transcript object.
3. Create a Report with the custom report type created.
4. Set Type = “Agent Left” as the report filter criteria.
what is the definition of agent left? is it a technical or agent issue?
It's not technical issue. None of the agent picked the Chat.
https://developer.salesforce.com/docs/atlas.en-us.api.meta/api/sforce_api_objects_livechattranscriptevent.htm