The issue you described happens when the spring, located inside the tube, moves out of position, it is generally not missing but just stuck inside the tube. The spring is not broken and should work if repositioned.
We apologize for any frustration you've experienced, and will absolutely send you a replacement. However, we have found that most people want to use their iWALK right away rather than wait for replacement parts to arrive, so we have created video instructions that show how you can put the spring back into the correct position.
If you're successful in repositioning the spring, then you can continue the assembly of your iWALK crutch. If not, or if you are not interested in attempting the repair, please contact us and we will send you your replacement part ASAP.