Default Membership cannot be assigned to users

Hello I am being helped by the developer Jack Kitterhing on a few issues.

However he takes time to respond and I have identified a new problem that I am hoping is easy to fix.

I have created a Defaut membership tier that I want to move all my registered users without a paid membership to.

The new membership "Basic" has been created and can be used in restriction rules BUT it cannot be assigned to any user.

Have I made a mistake?