During debugging, Toad creates two background sessions for handling debugger calls: Target and debug sessions. These sessions MUST be on the same instance as the main Toad session.
Requirements:
Have an additional entry in your TNSNames.ora file for the connected instance
Or
Connect directly to an instance of the cluster without letting the server assign one.
Toad accomplishes debugging as follows:
Queries the data dictionary to find the current instance name
Searches the TNSNames.ora file for the first entry that matches SERVICE_NAME or INSTANCE_NAME.
Uses the secondary TNSNames entry when creating the background debugger sessions.