If you have a right sized hole saw or a dowel you can shove in there, up to the screw then you can put on another caster on or the old caster if just its mounting was broken. If you don't have those, then a square peg in a round hole will do the same.
With those wood options it would be best to have pieces long enough to go past the screw above, so you can drill a hole for the screw to go through & be Support 1. Support 2 would be a metal plate at the bottom of the wood to shoulder on the leg's tube.
3rd option would be to use just a thicker metal plate on the bottom & fill the tube with washers the height of the caster's stem...if the caster has a hog ring or C clamp atop the stem, your top washer would need to be snug to the stem's diameter.
4th & easiest option is to get a new cup-mount caster. The cup just slides over the outside of the leg's tube & can be glued or drilled & screwed on.