I want to stop the trees to generate in air but i don't know how

This issue has been solved here ... https://mcreator.net/forum/85146/please-help-me-my-trees-are-floating?page=2

These instructions are conditioned to the size of yout tree structure ...

Set those marked values (x and z) to half of the size of your tree structure (x-y-z), in negative values. The Y could be 0, I just prefer to add a slight negative value ... just in case.

Why? Your tree is not centered with the structure block, meaning that the structure block is somewhere on top of the ground at the same level as the base of the tree. The x and z will position the tree at the center of where the structure block (not visible) should be, while the -1 or -2 will position the tree deeper in the grass (just in case, you can also change it to 0 or -3 if needed). And finally, surround the tree with structure void. Practice in order to get it right.

I could make a step by step if you need it. Will check later.