We have removed Akeeba Subscriptions from our products offering on Sunday. We are not going to provide a paid version or support for it any more. The next version (3.3, planned for two weeks from now) will be free of charge and include all features of the Professional version. From that point we're only going to be offering developer releases (free of charge) and no support.
Regarding your issue, you just need to install the latest release of Akeeba Subscriptions Core (the free version) twice. How do I know that you need to use the Core release and not the Professional release? It's simple, if you've seen it as many times as I have :) The error you mention comes from an old version of Akeeba Subscriptions, before we started selling a Professional release. It happens when you upgrade Admin Tools and Akeeba Subscriptions (or simply install a new version of the FOF library) without upgrading Akeeba Subscriptions.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
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