Install Ubuntu 20.04 on Virtualbox on Ubuntu 20.04

Problem: can't use sudo

When creating VM, check the option "Skip Unattached installation"

If you didn't check that and have completed the installation and find some problems, unfortunately, it's too late, you need to recreate VM and reinstall the OS. (If there is some easier way to fix it, please tell me, I would like to know)

Problem: Can't open terminal (worst thing ever)

1. Settings -> Language -> English (Canada)
2. reboot

Then you can open terminal. (I don't know why)
After that, even you switch to another language, the problem won't appear again.

Problem: The window is too small (like 800x800)

1. On the VM's top menu, select Devices -> Insert Guest Addition CD image
2. On the opened window's top right corner, click "Run software"
3. reboot
4. On the VM's top menu, select View -> Auto-resize Guest Display.

The time shown the top bar is in other langauges

edit /etc/default/locale to language you want

Comments

[Scott Reno]

Why use such an old version of Ubuntu?

[nullity]

I did try using Ubuntu 22, but I encountered some problems or errors and felt tired. I can't remember how I installed it and details about problems, but I met the first fatal error immediately at welcome page after installation.
I use Ubuntu 20 just because many people have used it, so it's easier to google solutions when facing problems.