I have an older machine, Asus I5, Radeon Rx 460. It came with Windows 10, which I do need to use the odd time, so I decided to dual boot and try Zorin 15.3. Initially when I first installed Zorin 15.3, I had no issues. Installed fine, booted up fine. Only issue was it didn't recognize my USB WiFi adapter. Turns out the one I had wasn't supported by Linux by default. Needed to disable secure boot to install the drivers for the WiFi adapter. Got it working, and reenabled secure boot. Then it stopped working. I figured I would have to leave secure boot disabled for it to work permanently and I wanted to avoid that, so I just bought a new WiFi adapter that's compatible with Linux out of the box. Plugged it in, and got the new one working no problem. Stupid me should have just left things as is, because everything was working at that point. If it's not broke, don't fix it.
I saw Zorin 16 came out, and decided hey maybe I should give this a go. Flashed it on a USB, booted via USB. It kept freezing unless I ran it in safe graphics mode. So I ran it in safe graphics mode to do the install. Installed fine. Rebooted, but it wouldn't boot into Zorin unless I added nomodeset in the boot up script. So I figured it was a graphics driver issue. Tried downloading the drivers for my graphics card and updating them, but couldn't get it to work. This is when I said screw it, I'll just go back to 15.3 where everything was working.
So I flashed and installed 15.3 again, however this time it wouldn't boot. Gave me error messages. At this point I figured I'd try a few different Linux distros to see if anything else would work - Ubuntu, Mint, and both 15.3 and 16 of Zorin. Nothing works. All these install fine but I can't boot into any of them. Ideally I'd like to stick with Zorin (prefer 16 but will settle for 15.3) if I can get it working.
I'd prefer to leave secure boot enabled (I know it is possible since I had everything working initially with it enabled). So here's where I'm at right now. I have 15.3 installed, but can't boot into it. The error message I get upon boot is:
error: /boot/vmlinuz-5.4.0-86-generic has invalid signature.
error: you need to load the kernel first.
I should note, whenever I do the install, it asks me to set a password to configure secure boot. I enter one. When I reboot, it doesn't prompt me to enter it. If I boot into Zorin via USB, that's when it prompts me to enter it to enroll the keys. I enter it, but still can't boot into Zorin regardless whether it enrolls the keys successfully or not.
Sorry for the long post, but I figured I'd have the best chance fixing this by giving as much information as possible.