Cannot move window to another workspace Ubuntu 18.04

Solution 1:

Try the following key combination:

Ctrl+Alt+Shift+Arrow key (Left/Right/Up/Down).

Solution 2:

Try:

• Super+Shift+Page Up
• Super+Shift+Page Down

https://help.ubuntu.com/stable/ubuntu-help/shell-workspaces-movewindow.html.en

Solution 3:

Some applications (for example TWS trading java platform from Interactive Brokers) would not allow to move it's main window with mentioned in other answers hot keys (even though all other applications allow to do so).

So here's the "Universal Method" for stubborn apps like this (tested in Gnome):

Hit "window" key once - it should show you "Activities overview" with all your apps tiled and you'll notice all your workspaces "preview" vertical bar on the right side of the screen.

From here you can drag-n-drop you app window to the desired workspaces in the "preview" vertical bar.

Hope it helps!