Can a spinal tap show something a mri cant show? are there any other test that can be done to explain burning sharp back pain? i had a mri of brain, neck and whole spine with explanation as to why and im in severe pain and cant even bend?

Yes. There are medical conditions that might cause findings in spinal fluid that do not show up on MRI, CT, or X-rays. If your doctor has advised lumbar puncture (LP, "spinal tap"), you should agree. It may sound frightening ("needle into the spine"), but LP is not very painful and has little risk of harm. Good luck.

As has been answered, a series of tests from the spinal fluid can reveal hematological, chemical, and metabolic, and immunological abnormalities not diagnosed with imaging, but unless you suffer an infection of the vertebral bodies or meningitis, might consider alternative tests, such as a bone scan, as the next step. Discuss fully with your doctor.