Armbian OS (Debian Linux) for Tinkerboard does not have 1368x768 resolution by default and I used 1280x720 for a while. Today I found very nice solution

• Open terminal window, type
  cvt 1280 800
  This should give you the modeline values to put into:
  /etc/X11/xorg.conf.d/20-modesetting.conf
•  Then add
  
  Section "Monitor"
  Identifier "HDMI-1"
  Modeline "1280x800_60.00"   83.50  1280 1352 1480 1680  800 803 809 831 -hsync +vsync
  Option "PreferredMode" "1280x800_60.00"
  EndSection
  
  
• Reboot Tinkerboard, new mode 1280x800 should appear in display settings.

PS: version for 1368x768 is :

• Open terminal window, type
  cvt 1368 768
  This should give you the modeline values to put into:
  /etc/X11/xorg.conf.d/20-modesetting.conf
•  Then add
  
  Section "Monitor"
  Identifier "HDMI-1"
  Modeline "1368x768_60.00"   85.25  1368 1440 1576 1784  768 771 781 798 -hsync +vsync
  Option "PreferredMode" "1368x768_60.00"
  EndSection
  
  
• Reboot Tinkerboard, new mode 1368x768 should appear in display settings.