can not install from usb with error cannot find grub_platform

After creating a bootable pen drive, you have to convert it to MBR. Insert the pen drive and use Gparted to find out whether the pen drive is detected as sdb or sdc etc. For this explanation, I'm assuming sdb.

sudo gdisk /dev/sdb
Enter r for transformation.
Enter g for converting GPT to MBR.
Enter w for writing to MBR partition.
Enter y to finalize and exit.
sudo fdisk -l /dev/sdb

This time, when you select "Ubuntu" when booting from the pen drive, wait for around one minute until the graphic screen shows up (coz there's a blank screen that persists for quite long). Everything works fine then.

This is probably caused by the fact that your laptop does not support UEFI.

Converting the USB drive partition table from GPT to MBR may help in that case.

See https://bugs.launchpad.net/ubuntu/+source/casper/+bug/1905491/comments/8