Of course the uTrack24 will work with an unbalanced signal!
If you only have an insert available (no direct out), you will have to join the tip and ring of the jack together, in order not to interrupt the signal in the mixer.
In that case screen and 1 signal wire (to the uTrack24) are connected to the sleeve, and the tip/ring connection of the jack carries the signal.
So you have to add an unbalanced attenuator.
If you really need 15 dB attenuation(!), I would add 18K.ohm between tip/ring of the jack and the (other) wire going to the uTrack24, and also add a 10K.ohm resistor between the signal wire and the sleeve of the jack. So the signal wire to the uTrack24 should be at the point where the two resistors are joined.