Hey @Jessica_LOPES
welcome to the forum
Can you explain please why you want to add the subtask to the project again? Because the subtask is basically already part of the project as you can see also in your screenshot it shows in list view.
Since you added the subtask also to the same project this is why it shows under the project itself as well basically like a normal task within the project + a subtask in the project
This might be helpful as well: Subtasks appear duplicated when specifically selected and moved to a different section/project in list view - #2 by Natalia