If you have multiple display and a AMD grahics card and want to expand the desktop to all displays you might get a message like :

required virtual size does not fit available size: requested=
(3200, 1200), minimum=(320, 200), maximum=(1920, 1920)

To solve this you have to run the AMD display configuration:

sudo aticonfig --initial

to re-initialize your configuration files. You might have to restart (I did)

Now run:

sudo amdcccle

and you’ll be able to configure dual display.

Go to Display Manager and choose Multi-display desktop with display(s)… in Multi-Display tab for both monitors.

Restart Ubuntu after this, Then disable the “Mirror monitors” option in the Ubuntu display settings.

You might have to move the display around in the Ubuntu display setting to get things right.