I've had many tests down. burning left upper back..no one knows what happening, going another test thoracic spinal stenosis. . what could it be?

Hard to say: Certainly hard to say without benefit of your records. A thoracic radiculopathy can cause upper back burning, as can a peri scapular problem such as a sub scapular bursitis, or something deeper inside such as a lung , stomach, or pancreas problem for the left side. A real detailed exam and appropriate studies can usually determine the cause.