you need to attach the towel bar to something substantial. Under typical usage it will eventually pull out if installed in drywall. spackling the hole and screwing into the same location will just pull right out again.

One way is to cut a hole in the drywall, slide a piece of wood into the wall cavity behind the hole and position it to span across the hole. Then screw to the wood. when it's installed patch the drywall around it.

Thanks for the reply!

The towel bars have all held up, except my son, who was 8 at the time, decided to use it as a pull-up bar :(.

Look at the other side of the wall behind the towel bar. If it is a closet, consider cutting an opening in that side of the wall to permit reinforcing that towel bar mount. In one bath, I cut out the drywall in the towel bar area, used plywood of the same thickness nailed to the studs and used wall paper in the entire bath to conceal the plywood.