Pick a first weight you could lift 2 or 3 more times. For a range of 8-12, choose a weight you could do about 14 times. Too light is fine: you go up quickly.
Work inside the rep range. When every set reaches the top of the range, add the smallest weight step next time: 2.5 kg on a barbell, machine or cable, 2 kg on dumbbells (5 lb in lb).
From your second session, Liftorra fills in last time. Every set arrives with last session's reps and weight. When every set hit the top of the range, the rows come prefilled one step heavier, at the bottom of the range. Your first session shows Set weight instead of a number, so type the weight you chose.
General guidance, not medical advice. If a movement hurts, stop and ask a qualified coach or physio.