What is the cause of low back pain, buttock pain, pain in left leg and left knee?

Radiculopathy . Sounds like a radiculopathy from nerve compression. Get referred to a neurologist, physiatrist, or spine surgeon with an updated mri.
Many things. Low back can come from many things: trauma, overuse, degeneration, infection, tumors. You are young, so muscular pain, overuse, or early degeneration is more likely. Buttock pain is commonly associated with low back pain. Leg pain may relate to spine nerves or be completely different. Knee pain in isolation is probably not from the back.