If you have a headless machine (ie: no monitor plugged into it) you’ll need to either install a “HDMI headless dummy connector” or you can install a Video Dummy Package:

sudo apt-get install xserver-xorg-video-dummy

Then add a configuration file for it:

sudo vi /usr/share/X11/xorg.conf.d/xorg.conf

and paste this (adjusting your desired resolution)

Section "Device"
    Identifier  "Configured Video Device"
    Driver      "dummy"
EndSection

Section "Monitor"
    Identifier  "Configured Monitor"
    HorizSync 31.5-48.5
    VertRefresh 50-70
EndSection

Section "Screen"
    Identifier  "Default Screen"
    Monitor     "Configured Monitor"
    Device      "Configured Video Device"
    DefaultDepth 24
    SubSection "Display"
    Depth 24
    Modes "1920x1080"
    EndSubSection
EndSection

Then reboot the machine and voilá

Source: Ubuntu 20.04 shows a black screen when connecting through Teamviewer, but I still have ssh access. How can I get back the desktop? – Ask Ubuntu