BEFORE YOU DO ANYTHING TO YOUR PANEL, BE SAFE AND SWITCH OFF THE MAIN SUPPLY. THAT SHOULD BE THE VERY TOP ONE IN YOUR PICTURE WITH THE BLUE STICKER. USE A TESTER ON THE TERMINALS OF THE OTHER BREAKERS TO VERIFY THEY ARE NOT ENERGIZED BEFORE DOING THE WORK.
What you call a double breaker is really called a “tandem breaker”. It occupies a single slot, has two breaker switches, and allows two circuits to be attached (to the same phase), each protected separately. What is above “new circuit” and on “car charger” are what is called double pole breakers. They are meant to supply both phases to a single circuit and protect that single circuit as a whole.
Panels have “slots” for breakers. The slots alternate what supply phase they are connected to. This is what allows a double pole breaker to attach to both phases (as it connects in two adjacent slots). A tandem breaker only connects to a single slot and thus both circuits will use the same phase (the location of the breaker in the panel dictates which of the two phases). Thus the two lines attached to “Current slot” use one phase, the two lines attached to “New Slot” use the other phase.
“Current Slot” provides two wires (to sense) and by virtue of it being on a tandem breaker, they are both on the same phase. This then also explains the error message you are getting… Sense needs to be supplied with 110/120V from each phase as a reference so it can observe what is happening to each phase. This is why they are complaining.
The “car charger” breaker is a double pole, providing two different phases, one to each connection. Thus the car charger sees 240V (as it should). The breaker above "“New Slot” is also a double pole breaker.
So you need to get a new, double pole breaker, like the one for the car charger, 15A capacity should suffice. From the picture it looks like your panel can use Schneider Square D breakers (or compatible). It can go right below “Current slot”, or right below “car charger”. Remove the red/black wires from the breaker in “current slot”, and then connect the red and black wire currently on the “Current Slot” to the two terminals of the newly installed double pole breaker. Finally, if you so desire you can remove the tandem breaker for “current slot”.
It is fairly customary to use one color for one phase, and the other for the other, consistently. Your first two double pole breakers have black/red in the same position. Notice the car charger uses two black wires so no telling which one is which. This is perhaps unfortunate, but it works just fine and you don’t want to call out an electrician to replace a black wire with a red one. It might be consistent, once connecting to the new double pole for sense, the keep black on top, red on bottom. If you re-use the tandem breaker in “current slot”, ideally you should use two red wires for whatever you plug in.