Patch holes in medium density fiberboard
I think there are two probable options here.

First off, the easiest solution may be  as 'Ed Beal' Suggests, and simply  move the bracket over a few inches and re-fasten.

The other solution, albeit not very "elegant" is often highly effective. Liberally coat a cut piece of wood just bigger than a toothpick, and just shorter than your hole is deep. Slide in the glued piece of wood, the insert and tighten down your screw.

The screw works to force and press the new wood into the hole and give the screw something to "bite" into. The friction and pressure of this new piece of wood is often enough to keep the fastener in place. The glue keeps it in long term.

It's not an excellent solution if you plan on repeatedly removing the fasteners, or you rely on the fasteners to be in tension often. In this case where you're just holding a bracket in place where the leg will be in compression, I think you can get away with it.