Can't put additional icons into toolbar

I'd like to customize the toolbar = put more icons into it, like f.e. cut,
copy, paste, delete, etc.
I've read that I need to right-mouse-click into an empty space on the
toolbar and then choose "customize". But nothing happens when I right-click
in an open area of the toolbar.
Any help would be very much appreciated.
Many thanks in advance.

Sigrid