Why can’t we put it on a rocket and aim it toward the sun?
Too heavy.
We can, but we shouldn't, because that'd be like putting barrels of crude oil on a rocket and aiming it toward the sun.
Coal ash has enough nuclear fuel in it to potentially produce more heat than the coal that was burned to generate the ash, due to naturally-occurring traces of uranium and thorium in the coal. So just imagine how much potential heat remains in spent nuclear fuel which has had only about 3% of its potential energy extracted.
Spent nuclear fuel is a natural resource, so why rocket it into the Sun where it'll be rendered absolutely useless?