>>7590073
You should be able to see the missing tail-fin on the left side of the plane (red). And your box is off. To know where the center-line of a box is, you can draw a X (blue) and run a mid-line between the it. You can see that your plane doesn't really match up with the box you drew. But your box was off anyways.
I still struggle with boxes so I tried "fixing" it but it was a bit sloppy. Anyways, You should watch >>7570693 videos, particularly the Industrial Design Fundamentals applied video, he mentions a rule of thumb, the further away the side of the box is, the smaller it should be.
So the two front sides of the boxes should be "taller" than the back side of the box where the tail is. The top corner one being the smallest one where the red one I marked.
Anyways your plane still looks good. Its just a lot of practice we need to do, Pretty much a lot of people in this thread have a far better understanding than I do as you can see from the vehicles they drawn.
>(Cant quote because it think its spam)
You could just try to see where you went wrong like I did digitally as well. The key is that your plane still looks good, it would have been a lot worse if it followed what your box shows.
I guess this is what they mean by drawing though the "form" or whateverr