How can I permanently set the screen resolution?

Hardware Support
21

That resolution's already added, since it's the resolution you're using right now. Not sure if that's causing the error.

Try it without the -r flag... it should attempt to use a resolution that you've set up that's closest to the highest frequency capability of the monitor.

So for instance, say you wanted to set it to 1600x900:
sudo xrandr --output HDMI-0 --mode 1600x900
-- or --
sudo xrandr --output HDMI-0 --mode 1600x900 --rate 60
-- or --
sudo xrandr --output HDMI-0 --mode 1600x900 --refresh 60

I'm not sure if sudo is needed here, but if it's not, it shouldn't be a problem.

The --output flag has several sub-flags:

--output <output>
      --auto
      --mode <mode>
      --preferred
      --pos <x>x<y>
      --rate <rate> or --refresh <rate>
      --reflect normal,x,y,xy
      --rotate normal,inverted,left,right
      --left-of <output>
      --right-of <output>
      --above <output>
      --below <output>
      --same-as <output>
      --set <property> <value>
      --scale <x>x<y>
      --scale-from <w>x<h>
      --transform <a>,<b>,<c>,<d>,<e>,<f>,<g>,<h>,<i>
      --off
      --crtc <crtc>
      --panning <w>x<h>[+<x>+<y>[/<track:w>x<h>+<x>+<y>[/<border:l>/<t>/<r>/<b>]]]
      --gamma <r>:<g>:<b>
      --brightness <value>
      --primary

... so you can set brightness, gamma, positioning, rotation, screen mirroring, etc. as well as the resolution and refresh rate.

So, for instance, say you wanted to mirror HDMI-0 to be the same as your main screen, and let's say eDP is your main screen:
sudo xrandr --output HDMI-0 --mode 1600x900 --refresh 60 --same-as eDP

Or, let's say your HDMI-0 is physically located to the right of your eDP screen, and you wanted different outputs on each screen:
sudo xrandr --output HDMI-0 --mode 1600x900 --refresh 60 --right-of eDP

Now, when you move your mouse cursor off the right-hand edge of eDP, it'll move onto HDMI-0, and when you move it off the left-hand edge of HDMI-0, it'll move onto eDP.

Let's say you've physically rotated HDMI-0 to the left (counter-clockwise) so it's vertically taller and horizontally smaller (photograph mode, rather than landscape mode):
sudo xrandr --output HDMI-0 --mode 1600x900 --refresh 60 --rotate right

Now you'll see your screen properly oriented, but taller than it is wide.

Let's say you've got it all set up the way you want, but the HDMI-0 monitor's looking a little dark:
sudo xrandr --output HDMI-0 --brightness 1.1
1 is default.

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