This seems to be a kernel bug. See the bug report below:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1930783
This issue comes in some Acer laptops as its BIOS contains two obsolete certificates. The error message is harmless and can be ignored. Some patch is available for Linux kernel to hide this error, but its not part of Ubuntu yet.
You might also want to consult this excellent tutorial for preparing the system for installation: