Halp

HTML

<form>
  <input type="checkbox" id="someId">
  <label for="someid">Some label</label>
  <button type="submit">Submit</button>
</form>
<form>
  <input type="checkbox" id="someOtherId">
  <label for="someOtherId">Other label</label>
  <input type="submit" value="Submit">
</form>
<form>
  <input type="checkbox" id="andAnotherId">
  <label for="andAnotherId">Another label</label>
  <button type="submit">Submit</button>
</form>

JavaScript

/* eslint no-restricted-syntax: 0 */
const allForms = document.querySelectorAll("form");

for (const form of allForms) {
  const agreementCheckbox = form.querySelectorAll(
    'form input[type="checkbox"]'
  );
  const submitButton = form.querySelectorAll('form *[type="submit"]');
  agreementCheckbox.forEach(checkbox => {
    checkbox.addEventListener("change", () => {
      submitButton.forEach(button => {
        if (button.getAttribute("disabled")) {
          button.removeAttribute("disabled");
        } else {
          button.setAttribute("disabled", "disabled");
        }
      });
    });
  });
}