You are seeing that error because you have
root=UUID=..., but no initrd line. The kernel doesn't have built-in support for
root=UUID=.... It has to be handled by an initrd.
- So you could just add an initrd line. Normally you should have an initrd generated during the install process.
Otherwise I think the problem is with your (Slackware) linux install, and you need to rescue it. OS install media often include very handy rescue systems. I strongly recommend obtaining such a rescue system when you install your OS, and keeping it around. (And check you would still have internet access, if you're not already confident about how to use the rescue system).
- Or you could try not using the UUID. Instead, specify the device node for the root filesystem directly, such as
root=/dev/sda1. You could put the UUID back after you finish bootstrapping / rescuing your system.
In your comment you present an interesting circular dependency. How to generate the initrd, if that process requires booting the exact same kernel? However I don't think that requirement makes any sense. (Consider the installer, kernel upgrades etc.) It should be quite possible from a matching rescue system, or when chrooted into the install from a newer rescue system...
You must be thinking of
mkinitrd_command_generator.sh. I see elsewhere an example of invoking it for a specific kernel.
Hopefully this means it isn't limited to the current kernel, because that would be a bit useless.