a.l.e. and utnik my most sincere gratitude for your help. Thanks to you everything is resolved. The problem of seeing the blank window was due to the dark mode in which you can work with Mac OS 10.14 Mojave. There are several solutions.
First.– From System Preferences> General go to clear mode when working with Scribus 1.5.5
Second.– Open in Applications> Utilities> Terminal and write in the window that opens "defaults write -g NSRequiresAquaSystemAppearance -bool Yes" without the quotes. Only the menu bar, the dock and certain parts of the system will appear in the dark mode, but not the applications, including Scribus 1.5.5 and the contents of the editing window will be displayed correctly. To eliminate this way of working, simply write to the Terminal "defaults write -g NSRequiresAquaSystemAppearance -bool No".
Third.– There is a small free application called NightOwl very useful to make all these changes with one click.