xscreensaver-command: can't open display

usage: xscreensaver-command -<option> This program provides external control of a running xscreensaver process. Version 5.36, copyright (c) 1991-2016 Jamie Zawinski <jwz@jwz.org>. The xscreensaver program is a daemon that runs in the background. You control a running xscreensaver process by sending it messages with this program, xscreensaver-command. See the man pages for details. These are the arguments understood by xscreensaver-command: -demo Ask the xscreensaver process to enter interactive demo mode. -prefs Ask the xscreensaver process to bring up the preferences panel. -activate Turn on the screensaver (blank the screen), as if the user had been idle for long enough. -deactivate Turns off the screensaver (un-blank the screen), as if user activity had been detected. -cycle If the screensaver is active (the screen is blanked), then stop the current graphics demo and run a new one (chosen randomly.) -next Like either -activate or -cycle, depending on which is more appropriate, except that the graphics hack that will be run is the next one in the list, instead of a randomly-chosen one. In other words, repeatedly executing -next will cause the xscreensaver process to invoke each graphics demo sequentially. (Though using the -demo option is probably an easier way to accomplish that.) -prev Like -next, but goes in the other direction. -select <N> Like -activate, but runs the Nth element in the list of hacks. By knowing what is in the `programs' list, and in what order, you can use this to activate the screensaver with a particular graphics demo. (The first element in the list is numbered 1, not 0.) -exit Causes the xscreensaver process to exit gracefully. This does nothing if the display is currently locked. (Note that one must *never* kill xscreensaver with -9!) -restart Causes the screensaver process to exit and then restart with the same command line arguments as last time. You shouldn't really need to do this, since xscreensaver notices when the .xscreensaver file has changed and re-reads it as needed. -lock Tells the running xscreensaver process to lock the screen immediately. This is like -activate, but forces locking as well, even if locking is not the default. If the saver is already active, this causes it to be locked as well. -version Prints the version of xscreensaver that is currently running on the display -- that is, the actual version number of the running xscreensaver background process, rather than the version number of xscreensaver-command. -time Prints the time at which the screensaver last activated or deactivated (roughly, how long the user has been idle or non-idle -- but not quite, since it only tells you when the screen became blanked or un-blanked.) -watch Prints a line each time the screensaver changes state: when the screen blanks, locks, unblanks, or when the running hack is changed. This option never returns; it is intended for by shell scripts that want to react to the screensaver in some way. See the man page for more details. For updates, check https://www.jwz.org/xscreensaver/