For beginners, it is more common to do sit-ups with low back pain. In fact, it is not low back pain, but muscle pain in the iliopsoas muscle of the waist. The reason is that when doing it, when lying down, the thigh and trunk are in a straight line, which causes the iliopsoas muscle to be elongated; When doing it, the iliopsoas muscle is too hard and the abdominal muscles are reduced (anatomical knowledge, the iliopsoas muscle starts from the lumbar spine and pelvis, as for the thigh and femur). So it causes the pain of iliopsoas muscle and makes you feel low back pain.
Solution: Bend your legs and do sit-ups. When lying down, bend your legs, land your feet, keep your thighs and torso at a certain angle, and then stick to it. Your abdomen will get better exercise.
I wish you success in fitness and hope to adopt it.