So, you downloaded and installed the new macOS 10.13 High Sierra after checking that most/all of your audio software and plugins are now compatible... and then discover that Apple's own DAW, GarageBand, can't load any of your 3rd-party plugins.
If you've come across the warning stating: "The Audio Unit "XXX" could not be loaded for your project, and may need to be updated..." then before you throw anything at your Mac's computer screen, breathe and read on to discover a temporary solution that'll help you get around this security/sandboxing issue.
We're sure Apple will provide an update in the near future, so an upcoming macOS or GarageBand update might well be the best solution. And if you use Logic Pro X or any other DAW your plugins should load absolutely fine there. This is just a GarageBand and macOS High Sierra issue. If you're experiencing this issue like many others in the Apple Discussions forum here then you can try this trick courtesey of Two notes Engineering:
Our support manager at Ask.Audio, Christian Labbé, tested the above on macOS High Sierra with GarageBand and noted it works perfectly, but you will need to carry out this process every time you launch GarageBand.
But at least this is a band-aid fix over a security issue that looks like it should be very easy for Apple to solve in a coming update.
[Via: Two-Notes Engineering]
Become a master of GarageBand for iOS and Mac: https://ask.audio/academy?nleloc=application/garageband