Membership 2: Registration is currently not allowed.

People are not longer able to register accounts using Membership 2 on my Multisite network. It used to work but now it's not working. I've checked Network Settings to ensure the "Logged in users may register new sites." setting is that means people should be able to register right? But when they click the button for the Membership subscription to purchase it, it displays: "Registration is currently not allowed."

See for yourself:

I have already enabled support access if you want to jump in.

I look forward to a prompt reply.