When it try to start a graphical application from a terminal and it fails returning only an error of type cannot open display is very likely to be an environment variable error. In Unix-like systems for each Xorg server must be a logical display identifier of the form :0, :1, ..., :0.0, :0.1, ..., :1.0, :1.1 and so forth.
** (PROGRAM_NAME:PORT): CRITICAL **: Failed to parse arguments: Cannot open display:
the most frequent:
(PROGRAM_NAME:PORT): Gtk-WARNING **: cannot open display: VALUE
Error: cannot open display: VALUE
indicate that the VALUE logical display cannot be accessed ...