Ubuntu – Just upgraded to Ubuntu 18.04 and can’t use compiz grid window resize commands

Install compizconfig-settings-manager . Search for "compiz" in the Unity Dash, or start with ccsm from the commandline.

Warning

CompizConfig Settings Manager (ccsm) is an advanced tool, and not all of its options are completely compatible with Unity. As such, CCSM has been known to sometimes break users' desktops. Please use caution and know that you are taking a risk by following this answer or any other answer that recommends the use of CCSM. In the event that following this advice does break Unity, please see this question for instructions on how to reset it.

Find the Grid plugin:

Click on the key binding you want to change, click on Grab key combination, and press the keys you'd like to use.

The terms we are discussing here are "keyboard shortcuts" and "keybindings". They are useful when searching for help.

First, run xev in a terminal to see if the key works.
If it works, open Settings > Devices > Keyboard for a list of shortcuts. If it says Alt without specifying right or left they should both work. Replace any that specify a side and make sure it only says Alt.

If you have trouble making the settings do that, install dconf-editor

sudo apt install dconf-editor

and launch it from the terminal

dconf-editor 

Locate the search icon and type keybinding and look through the paths for what you need. In your example navigate to

org/gnome/desktop/vm/keybindings/close 

If the default value is on it should be set to

['<Alt>F4']  

If it's not, switch default to off and enter exactly the above as a custom entry.

My Alt shortcuts seem to be the same for each side, so maybe yours got reconfigured somehow.