Why can't I seem to levitate using the /effect command?

Your old command should work. Maybe check your setup?

/effect @s levitation 1000 1

Make sure you’re not flying while trying to use the levitation effect. If you are, you will get the particles and icon, and the effect will show up when you press Z, but the effect will not make you levitate. You can rule out the possibility of flying by entering survival mode with /gamemode s.

The effect ends after 0.5 seconds with the given command. By the time you'd like to test it, it's over.