Comment 4 for bug 1767276

Go to the hyperlink which Steve has mentioned and install the kernel. Then boot choosing the newly installed kernel option and go to recovery mode and upgrade your grub. Again reboot but choose the 4.15 kernel. And update. the shim package is installed.

I dont know the technical details as to what was the problem but my 18.04 ubuntu is working fine now.

The apt-get upgrade option does not mention the error code.