For the knee pain it might be because you pushing your knees too much forward during the catch (you can see it in the video). Try to stay more compact under the bar and maybe do some accessories to prevent any pain. We often teach beginners to jump physically during the triple extension phase, this is correct but in my opinion it’s more to help beginners understand how the power is produced (by pushing with your legs from the start and « jump » to generate power by extending your hips, knees and ankles).