Jellyfish don’t have a heart no, lots of animals don’t have hearts. Humans need a heart to send blood round our body, and we need to send blood round our body to carry oxygen to our cells.
In jellyfish their skin and body layers are so thin that their cells can just take in oxygen from the seawater that is surrounding them because it diffuses into the cells, and they can send the waste gasses straight back into the water out of the cell. The cells are all close enough to the water that they can do this. This means that they don’t need blood or a heart. It wouldn’t work for us because our bodies and skin are much thicker, the oxygen would only be able to get to the first layer of cells so the cells of our insides would die.
Jellyfish are very strange creatures, a lot of their body is actually just made up of jelly goo that isn’t even live cells so those parts don’t even need oxygen.