Live USB error "you need to load the kernel first"
If you get to a grub prompt, it means that grub can't find the boot files that it expects. The sequence of commands to load the files and boot when grub doesn't do that for you goes something like this. First, find all partitions that grub sees:
grub> ls (hd0) (hd0,msdos2) (hd0,msdos1)
This lists disks and partitions on the disks. One of these partitions holds your Linux system. Say it is (hd0,1). Then do:
grub> set root=(hd0,1) grub> linux /boot/vmlinuz-4.15.0-45-generic root=/dev/sda1
Replace (hd0,1), the version number and the partition (/dev/sda1) by what is valid for your system. In the case of vmlinuz you can just type vmlinuz- and press Tab.
grub> initrd /boot/initrd.img-3.13.0-29-generic
The version string should be identical to the one for vmlinuz.
grub> boot
should now boot up your system.
When that happened to me I just had to disable secure boot and it worked.
What do I need to do to, "load the kernel first"?
Using "Startup Disk Creator" to create thumb drive, I was unable to boot using the thumb drive, with the same recommendation.
I have a slightly different answer.
On my 14 year old Dell desktop running Lubuntu 19.10 I used "Startup Disk Creator" to load "ubuntu-19.10-desktop-amd64.iso" to a thumb drive.
The first experiment on my new (delivered last month) Dell laptop gave a similar error, with the identical offering ... "load the kernel first".
Reading the comments above, I reviewed my usb ports and found my Logitech mouse appeared to be the only usb connection.
Disconnecting the usb mouse eliminated the error.
No grub commands needed.