Its a 'fake block' and can't be docked.
In reply to Gareth J Barnard
Odp: Re: How to dock (or hide) the "Add a block" component
by Janusz Jabłonowski -
Thank you for your answer!
Since it is not a real block, is there a way to "promote" it to a regular (dockable) block? Or to remove it entirely? Anything that would free the left column .
Best regards!