I encountered the following error message when I ran the command plot(iris) on a local machine with Debian 10:

Error in .External2(C_X11, d$display, d$width, d$height, d$pointsize,  : 
  unable to start device X11cairo
In addition: Warning message:
In (function (display = "", width, height, pointsize, gamma, bg,  :
  unable to open connection to X11 display ''

Based on some of the advice on the internet, I have changed the display by issuing the following command in the terminal:

$ export DISPLAY=127.0.0.1:7.0

In R I issued the command capabilities() but it indicated that X11 is false and Cairo is true.

Update

After issuing the command x11(), the output is as follows:

No protocol specified 
Error in .External2(C_X11, d$display, d$width, d$height, d$pointsize,  :
    unable to start device X11cairo 
In addition: Warning message: 
In X11() : unable to open connection to X11 display '

Can any expert help me out here?