Prepare BIOS for Ubuntu installation
Reboot the computer while clicking on F2.
In the BIOS, change the following settings:
Security -> Secure Boot -> Disabled
To start from a USB key, insert the key and when the computer starts click on F12.
If the grub menu does not display on startup, click on ESC when the logo is first displayed.
Then if the grub loader goes directly into grub command line, type "normal" then wait.