Solution for Magento 2 Community Edition (CE / Open Source)
If you get the error "Product Key is empty or invalid. The extension has been automatically disabled." while saving Magefan extension on Magento Community edition and don't see any Product Key field, as on the screenshot:
Sometimes instead of the real customer IP address, you can see 127.0.0.1 or some other wrong customer IP in your Magento 2 Admin panel. It can be due to proxies, like Varnish, Cloudflare, that may be enabled for your Magento 2 store.