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Hi @lphoffmann,
Your best option here is some custom JavaScript. All fields rendered in a field group are targetable using the field keys or names so you can load a simple piece of jQuery on the page that initially checks to see if the required field is checked and toggles the disabled attribute on form button.
If you only need to prevent the form from being submitted, however, you could rely on ACF’s validation and require the checkbox field be checked. This won’t disable the form button but it will prevent the form from being submitted.
Cheers,
Phil