The answer is really simple. You should be able to do that unless the user-story you want to convert already has sub-tasks under it. You can not convert it to a sub-tasks with sub-tasks underneath.
So check if there is any sub-tasks, and if there is, probably you shouldn’t be changing the user-story to a sub-task. The user-story has sub-tasks, meaning that it was not that small. So rethink before deleting the sub-tasks and converting.