System Extension Blocked - but can't allow extension

There is a workaround for this. It isn't a great workaround, security-wise, but it does get you back to the behavior of macOS 10.12. Basically, you can turn off the security feature requiring user approval of Kernel Extensions.

First, boot into Recovery Mode by rebooting and pressing and holding command-R as soon as you see the Apple logo. On my machine, I had to hold command-R for quite a while, at least 60 seconds.

Eventually you will see a screen that gives you a couple of options, including reinstalling or running Disk Utility. If you look at the top of the screen, where the Apple menu would ordinarily be, you will see a "Utilities" drop-down menu. Click on that and select Terminal. A window will open up that allows you to type text-based terminal commands.

You will see a "$" prompt in the terminal window. Type the following:

spctl kext-consent disable

then press Return. You should see:

Kernel Extension User Consent: DISABLED

Please restart for changes to take effect.

Then reboot your Mac, and you should be good. You will no longer see the notification panes telling you a Kernel Extension was blocked; they will all be automatically allowed, just as they were in macOS 10.12.

I would recommend waiting until 10.13.1 or 10.13.2 to see if the issue gets fixed, and then undoing this fix by repeating the same steps, but typing

spctl kext-consent enable

instead.

For more information, see the following website:

User Approved Kernel Extension Loading… – Pike's Universum