xrandr "cannot find mode" although mode is added and shows on xrandr

I'm trying to use my tablet as an extended monitor for my laptop, following Prometheus's answer here.

I got to the point where I'm supposed to output the mode with VGA-0, but xrandr "cannot find mode."

$ gtf 1280 1024 60
    # 1280x1024 @ 60.00 Hz (GTF) hsync: 63.60 kHz; pclk: 108.88 MHz
  Modeline "1280x1024_60.00"  108.88  1280 1360 1496 1712  1024 1025 1028 1060  -HSync +Vsync
$ xrandr newmode "1280x1024_60.00"  108.88  1280 1360 1496 1712  1024 1025 1028 1060  -HSync +Vsync
$ xrandr --addmode VGA-0 1280x1024_60.00

At this point, prompting $ xrandr returns

Screen 0: minimum 8 x 8, current 1366 x 768, maximum 16384 x 16384
VGA-0 disconnected (normal left inverted right x axis y axis)
LVDS-0 connected primary 1366x768+0+0 (normal left inverted right x axis y axis) 344mm x 194mm
   1366x768       60.0*+
HDMI-0 disconnected (normal left inverted right x axis y axis)
  1280x1024_60.00 (0x2a6)  108.9MHz
        h: width  1280 start 1360 end 1496 total 1712 skew    0 clock   63.6KHz
        v: height 1024 start 1025 end 1028 total 1060           clock   60.0Hz

So there is a mode called 1280x1024_60.00. But now, trying to output that mode, I get

$ xrandr --output VGA-0 --mode 1280x1024_60.00
xrandr: cannot find mode 1280x1024_60.00

Which I don't understand because it looks like mode 1280x1024_60.00 has been successfully added.

Any advice or comment is appreciated. Thanks in advance.

On newmode, 1280x1024_60.00 is enclosed in double quotes, whereas on addmode and output it isn't. Use double quotes on those two commands too. At least, this is the correct syntax. An additional problem is that you are trying to add a mode to a disconnected port "VGA-0 disconnected (normal left inverted right x axis y axis)" which won't work. The port has to be connected first.

Judging from what the post says, you're probably using an incorrect port though. Your VGA-0 port is for connecting an actual monitor through a 15-pin VGA connector.