completion-auto-help

completion-auto-help is a variable defined in `minibuffer.el'.
Its value is

t

Documentation:
Non-nil means automatically provide help for invalid completion input.
If the value is t the *Completion* buffer is displayed whenever completion
is requested but cannot be done.
If the value is `lazy', the *Completions* buffer is only displayed after
the second failed attempt to complete.

You can customize this variable.