Constant NO_TUNING_MESSAGE 

pub const NO_TUNING_MESSAGE: &str = "franka target control: this session has no live tuning; \
     it is the Cartesian interface's on the impedance backend.";

The message of the FrankaError::InvalidOperation a live-tuning call returns on a session whose interface or backend has none: LiveTuning describes the impedance law of the Cartesian interface, and nothing else runs one.