double click to display slide menu!!

Where do you post ToggleSideMenu notification ?

When sideMenu is open, what should be the value of the constraint ?

Could you add print statements to trace :

    @objc func toggleSideMenu(_ sender: Any) {
        if sideMenuOpen {
            print("sideMenuOpen, former constraint", sideMenuConstraint1.constant)
            sideMenuConstraint1.constant = 236
            sideMenuOpen = false
        } else {
            print("sideMenu NOT Open, former constraint", sideMenuConstraint1.constant)
            sideMenuConstraint1.constant = 0
            sideMenuOpen = true
        }
        print("animation will begin")
        UIView.animate(withDuration: 0.3) {
            self.view.layoutIfNeeded()
        }
    }

Also, for testing purpose, remove animation temporarily

    @objc func toggleSideMenu(_ sender: Any) {
        if sideMenuOpen {
            print("sideMenuOpen, former constraint", sideMenuConstraint1.constant)
            sideMenuConstraint1.constant = 236
            sideMenuOpen = false
        } else {
            print("sideMenu NOT Open, former constraint", sideMenuConstraint1.constant)
            sideMenuConstraint1.constant = 0
            sideMenuOpen = true
        }
        print("layout will begin")
        // REMOVE FOR TEST UIView.animate(withDuration: 0.3) {
            self.view.layoutIfNeeded()
        // REMOVE FOR TEST}
    }

The original code you have shown, works as expected. So, you may be doing something wrong when posting the notification.

The behavior is something often found when you update UI components in the background thread. Aren't you posting the notification in the background thread?

Or you can try this code:

    @objc func toggleSideMenu() {
        DispatchQueue.main.async {
            if self.sideMenuOpen {
                self.sideMenuConstraint.constant = 236
                self.sideMenuOpen = false
            } else {
                self.sideMenuConstraint.constant = 0
                self.sideMenuOpen = true
            }
            UIView.animate(withDuration: 0.3) {
                self.view.layoutIfNeeded()
            }
        }
    }

Another possibility is your constant. The github code uses -240 to close and 0 to open. But you use positive number 236 to close.

Have you really set up your constraint as when the constant is 236 it's closed and when 0 open ?