'arrow function syntax (=>)' is only available in ES6 (use esnext option).

example.js

function disabledCheckBoxes(elements) {
    document.querySelectorAll(elements).forEach(element => {
        element.setAttribute("disabled", "disabled");
    });

}

function enabledCheckBoxes(elements) {
    document.querySelectorAll(elements).forEach(element => {
        element.removeAttribute("disabled");
    });