Upgrading ubuntu kernel with ukuu - invalid signature

I was following the RX590 Ubuntu 18.04 Setup Guide and am stuck at upgrading the kernel with ukuu.

I’ve tried installing various 4.20 kernels but when I reboot grub spits out an error saying the kernel has an invalid signature and I’m forced back to 4.15.0-45-generic.

This is a dual-boot setup with Windows10 on an nvme drive Ubuntu on a separate SSD. The Ubuntu install was a fresh 18.04.1 install followed by an upgrade to .2 before trying to follow the above guide.

Another odd behavior that I’ve noticed is that after kernel upgrade the system starts up at a grub prompt. When I type exit and hit enter, the system hangs. Restarting the machine gets me to the grub boot menu.

Any guidance appreciated. Thanks.

apt update && apt upgrade from the cli on next boot with 4.15? I haven’t seen that one before. Bad overclock?

Remember to disable secure boot in the uefi. The upstream kernels are not signed and won’t work with secure boot.