scrollView not working in stackView, How to programmatically scroll a stackView

Couple things...

First, not directly related, but it's a good idea to constrain elements to the view's Safe Area.

Second, we want to constrain positioning (Top / Leading / Trailing / Bottom) of the scroll view's subviews to the scroll view's Content Layout Guide (in this case, the only subview will be your stack view -- your "formula views" will be subviews of the stack view). Constrain sizing (in this case, the stack view's width) to the scroll view's Frame Layout Guide.

Third, if a UI element is the width of its superView, it's better practice to use Leading and Trailing constraints, rather than Width and CenterX constraints.

So, here's your class with some changes -- review the comments in the code:

class ViewController: UIViewController {
    
    let scrollView = UIScrollView()
    
    let formulas = ["", "", "","","", "", "",""]
    
    let formulasStackView: UIStackView = {
        let stackView = UIStackView()
        stackView.backgroundColor = #colorLiteral(red: 0.2587976158, green: 0.2588401437, blue: 0.258788228, alpha: 1)
        stackView.axis = .vertical
        stackView.alignment = .fill
        stackView.distribution = .fillEqually
        stackView.spacing = 5
        return stackView
    }()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        setupScrollView()
        addFormulas()
    }
    
    func setupScrollView(){
        scrollView.translatesAutoresizingMaskIntoConstraints = false
        formulasStackView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(scrollView)
        
        // always a good idea to respect the safe-area
        
        let g = view.safeAreaLayoutGuide
        
        NSLayoutConstraint.activate([

            scrollView.topAnchor.constraint(equalTo: g.topAnchor, constant: 0.0),
            scrollView.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 0.0),
            scrollView.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: 0.0),
            scrollView.bottomAnchor.constraint(equalTo: g.bottomAnchor, constant: 0.0),

            // use Leading and Trailing anchors instead
            //scrollView.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
            //scrollView.widthAnchor.constraint(equalTo: view.widthAnchor).isActive = true
            
        ])
        
        scrollView.addSubview(formulasStackView)
        
        let cg = scrollView.contentLayoutGuide
        let fg = scrollView.frameLayoutGuide
        
        NSLayoutConstraint.activate([
            
            // let's "inset" the stack view with 5-pts "padding" on all four sides
            formulasStackView.topAnchor.constraint(equalTo: cg.topAnchor, constant: 5.0),
            formulasStackView.leadingAnchor.constraint(equalTo: cg.leadingAnchor, constant: 5.0),
            formulasStackView.trailingAnchor.constraint(equalTo: cg.trailingAnchor, constant: -5.0),
            formulasStackView.bottomAnchor.constraint(equalTo: cg.bottomAnchor, constant: -5.0),
            
            // we have 5-pts on each side of the stack view, so
            //  constrain its width minus 10-pts
            formulasStackView.widthAnchor.constraint(equalTo: fg.widthAnchor, constant: -10.0),
        ])
        
        // these are wrong
        //formulasStackView.centerXAnchor.constraint(equalTo: scrollView.contentLayoutGuide.centerXAnchor).isActive = true
        //formulasStackView.widthAnchor.constraint(equalTo: scrollView.widthAnchor).isActive = true
        //formulasStackView.topAnchor.constraint(equalTo: scrollView.topAnchor).isActive = true
        
    }
    
    func addFormulas() {
        for index in 0..<formulas.count {
            addFormulaView()
        }
    }
    
    func addFormulaView() {
        let formulaView: UIView = {
            let view = UIView()
            view.backgroundColor = #colorLiteral(red: 0.1882152259, green: 0.1882481873, blue: 0.1882079244, alpha: 1)
            view.layer.cornerRadius = 6
            view.translatesAutoresizingMaskIntoConstraints = false
            return view
        }()
        formulasStackView.addArrangedSubview(formulaView)

        // views added as arranged subviews automatically get this set to false
        //  so not necessary
        //formulaView.translatesAutoresizingMaskIntoConstraints = false

        // all we need to do is constrain the height
        formulaView.heightAnchor.constraint(equalToConstant: 200).isActive = true
        
        // do NOT add either of these constraints
        //formulaView.leadingAnchor.constraint(equalTo: formulasStackView.leadingAnchor, constant: 5).isActive = true
        //formulaView.trailingAnchor.constraint(equalTo: formulasStackView.trailingAnchor, constant: -5).isActive = true
    }
}