If they used their own dev license then you’ll need to purchase your own. Even if the site had a separate license you’d need to know who purchased it as it would be tied to that email address. No license entered could be due to several things. It could have been removed manually, or there are cases where it will be removed automatically like if the domain name changes or you move hosting. The regular support team may be able to help you more. https://www.advancedcustomfields.com/contact/
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