I am 24 years old with unknown cause of elevated liver enzymes for more than 6 months with negative ultrasound can I use milk thistle as a liver tonic?

Yes. However, it is unlikely to benefit you; most studies show no value. Health doesn't come from taking pills. You need to discover the cause. I trust you've been tested for hepatitis b and c, hemochromatosis, wilson's, autoimmune hepatitis, and antitrypsin deficiency. Elite athletes tend to run high transaminases & that's healthy. If you're sedentary, think about a new focus on fitness instead.
Ask your doctor. I don't know about liver tonic but elevated liver enzymes are not normal. You should talk to your doctor to find out the causes. Some causes may be due to hepatitis, fatty changes related to diet or obesity, mediations or toxins, gallstones, disease of the pancreas, etc.