You can only hit the tree once.

First, you need to load the catapult. If you've already used it, you'll need to reload.

How do I reload?

If you're not hitting the tree, maybe the catapult isn't aligned properly.

You'll need to aim the catapult.

How do I aim it?

From its initial setting and from the left side, pull the "primitive art" twice. (If you've moved the catapult from its initial setting, push it so that it's pointing as far left as it can go. Now pull it from the right side three times.)

Now, push the rock from the ledge above, and you should hit the tree.