Gfilui

I've just installed Ubuntu MATE 18.04 on a laptop and have a dual monitor setup, both displays worked out of the box and the native display even went off on boot. I now need to use 1366x768 resolution on one of the displays but it is not an option. I've added a resolution but the resulting resolution is not correct I accidentally added 1368x768

Trying again gives error:

cvt 1366 768 60
sudo xrandr --newmode "1366x768_60.00" 85.25 1366 1440 1576 1784 768 771 781 798 -hsync +vsync
X Error of failed request: BadName (named color or font does not exist)
 Major opcode of failed request: 140 (RANDR)
 Minor opcode of failed request: 16 (RRCreateMode)
 Serial number of failed request: 31
 Current serial number in output stream: 31

To add a new mode to an output requires several commands in sequence, like so:

# create and name a new mode: "1366x768-0"
xrandr --newmode "1366x768-0" 75.61 1366 1406 1438 1574 768 771 777 800 -hsync -vsync

# attach the new mode to an output (VGA1)
xrandr --addmode VGA1 "1366x768-0"

# turn on the output using the new mode
xrandr --output VGA1 --mode "1366x768-0"

The reason for the error messages in the question is mode parameters cannot be changed. Use a new name (such as "1366x768-1") or remove the added mode. The sequence of commands to remove a mode follow:

# turn off the output
xrandr --output VGA1 --off

# detach the mode from the output
xrandr --delmode VGA1 "1366x768-0"

# remove the mode
xrandr --rmmode "1366x768-0"

Changes to xrandr don't survive reboot. To make settings persist, ubstitute parameters as appropriate for your setup and save into a script in /usr/local/bin and add it to Startup Applications. Then it will be run every time you log in. You might like to add it at boot up, rather than login.