While installing Ubuntu using a flash drive, you might get an error saying : /casper/vmlinuz not found. You cannot proceed with the installation until you correct it. To do this, you need to rename a file on the flash drive. You could log into another OS on your computer (if you have one) or use another computer.
1. On the flash drive, go to the directory vasper
2. Find a file named vmlinuz.efi
3. Rename that file to vmlinuz
4. Restart and boot using the flash drive. Now you'll be able to proceed with the installation.