I ran into this one today while working on a personal project. I had SSH access to a Mac Mini running Big Sur that I didn’t have hooked up to a monitor or keyboard. After digging around, I found I could enable VNC access via these commands.
sudo /System/Library/CoreServices/RemoteManagement/ARDAgent.app/Contents/Resouces/kickstart \ -activate -configure -access -on \ -configure -allowAccessFor -allUsers \ -configure -restart -agent -privs -all
If that doesn’t work, this will set a custom password not tied to any user account:
sudo /System/Library/CoreServices/RemoteManagement/ARDAgent.app/Contents/Resources/kickstart \ -activate -configure -access -on \ -clientopts -setvnclegacy -vnclegacy yes \ -clientopts -setvncpw -vncpw mypasswd \ -restart -agent -privs -all
When you’re done (since you don’t want to leave it enabled 24/7 for security reasons), disable with:
sudo /System/Library/CoreServices/RemoteManagement/ARDAgent.app/Contents/Resources/kickstart \ -deactivate -configure -access -off