If you've received this error, it is asking you to reconsider the placement of the red and black power wires. Each power wire is sitting on the same phase (or leg) of the panel. The main reason that we ask you to install on a 240V/dual-pole breaker is to better ensure that the two wires are placed on both phases of the panel. Avoid 120V or single-pole tandem breakers as each slot is powered by the same phase of the panel.
Although the Sense unit only draws about 4 watts, or less than 1 Amp, it still needs to monitor the voltage across both phases (A & B) of the panel. For that reason, we always recommend installing on the smallest 240V/dual-pole breaker available.
If you are installing in a single-phase (not split) panel, then this result would be expected. Please indicate this on the Phase type page of the Sense Pro app.