Does it mean that i have low testosterone because i can't grow back hair?

No: Many men don't grow much hair on their backs. The amount of body hair a man grows is primarily genetically determined. If you're the only male in your family (both your mom's and dad's families) without much hair on your back, asking your doctor to check a testosterone level may be reasonable.

No: If you're getting good erections, you've got enough testosterone and you're probably expressing your full genetic potential for body hair. Only a minority of guys have back hair, and they tend to go bald faster. I hope you're happy with whatever you happen to have. Best wishes.