Expected struct `std::boxed::Box`, found `i32`

Question

I am trying to instantiate an object of a BSTNode struct. But when I try to use the insert method I've implemented I am getting an error saying Expected struct 'std::boxed::Box' found 'i32', but inside the function my parameters are '&mut self' and 'i32', and to use the func from main I do something like that:

 let mut root: Option<Box<BSTNode>> = Some(Box::new(BSTNode {
        value: 0,
        left: None,
        right: None,
    }));
        root.insert(15);

but the insert func generates the error above.

BST Insert func:

pub fn insert(&mut self, val: i32) {
        if val <= self.value {
            match self.left {
                //If there are no nodes on the left, insert a new heap-allocated BSTNode holding the value passed to the fn
                None => {
                    self.left = Some(Box::new(BSTNode {
                        value: val,
                        left: None,
                        right: None,
                    }))
                }
                //If there's a node found on the left, recursively call the fn again on the node found.
                Some(ref mut node) => node.insert(val),
            }
        } else {
            match self.right {
                //If there are no nodes on the right, insert a new heap-allocated BSTNode holding the value passed to the fn
                None => {
                    self.right = Some(Box::new(BSTNode {
                        value: val,
                        left: None,
                        right: None,
                    }))
                }
                //If there's a node found on the right, recursively call the fn again on the node found.
                Some(ref mut node) => node.insert(val),
            }
        }
    }

BSTNode struct:

pub struct BSTNode {
    pub value: i32,
    //Hold a possibly-nullable pointer to a heap-allocated object -BSTNode
    pub left: Option<Box<BSTNode>>,
    pub right: Option<Box<BSTNode>>,
}

What am I missing? why can't I use the insert func properly?