Your condition is mainly due to the weakness of biceps brachii. You can use 15 kg with one hand and 30 kg with both hands at a time, but your weight is 72 kg! Compared with the load of 72kg, your biceps lack strength and endurance.
Suggestion: Don't rush for success, first start the static training of two arms, that is, hold the horizontal bar backwards with both hands, the grip distance is less than the shoulder width, and pull the bent arms to the front and back of the chest for 20 seconds before slowly recovering; After getting off the bar, pat and shake the biceps brachii for about 30 seconds (basically 10- 15 deep breaths), and then do the second action, 10 times as a group, 4 groups; You can do pull-ups when you don't feel a strong burning sensation at both ends of your arm when you practice all day. The first and second groups can hang 5 kg barbell pieces (overload, give yourself a positive psychological hint) to make 6 pieces, the latter two groups can do it with the help of their waist and hip companions, and the third and fourth groups can make 8 pieces with the help of their waist and hip companions.
Because it usually takes 48 hours to recover after deep muscle stimulation, no matter how hard you work, you must practice every other day. In this case, you can easily achieve your goal in 10- 14 days (unless you can't afford to gamble, that is, the effect of practicing every day will not be good, and it is difficult to achieve it in 7 days, so it is called scientific training).
Ensure nutrition and rest (muscle recovery) on the premise that training is not discounted.