I can confirm, I am facing the same issue on all macs upgraded to High Sierra. None of the suggested workarounds work on my side. I tried with the mouse but I remember mouse clicks in Rocksmith did not work in the previous MacOS either. Now the keyboard responds only with the sound feedback indicating a wrong key regardless of which key is pressed.
I don't want to contact the vendor of the software individually as it appears to be a common problem. Ubisoft should provide a patch asap.
I don't have a straight solution for this but will tell a story.
I have a 2006 imac that I put a new hard drive and power supply in a few years back. Worked fine and was an attractive piece of hardware.
One day, I upgraded my iphone to the new IOS and whamo, it stopped synching with the imac. Couldn't revert. Brought both to the apple store and you know what their suggestion was...buy new hardware. Yeah. Sure. Shortly thereafter, Steam stopped working on the imac. Wasn't supported.
So, I soon found out for myself what the best thing about a mac is. It can run windows. I'm using the same exact 2006 machine, running steam, itunes, etc. perfectly with windows 10 and it won't do a damned thing if I run it in MacOS. Added bonus...when you run steam on Windows, you can play Rocksmith 2012 and RS2014 in case you're missing the Clapton and Seth Chapla songs...
Anyway, long story but you can use the bootcamp assistant tool within your mac and install windows 10 and grab all the support drivers with relative ease. Then play rocksmith until your heart's content while you wait for Apple or Ubisoft or Steam to figure out that it's broken and volunteer to fix it.
Good luck!
Going into Steam and verifying files solved the problem -- here are the instructions from Uplay Tech support: 1. Launch Steam.
2. Select the Library tab at the top of the window.
3. Right- or (if using a Mac) control-click the game title in the list on the left.
4. Select "Properties".
5. On the Local Files tab, select "Verify Integrity of Game Cache".
Then restart Rocksmith and the keyboard commands work. Problem is I have to do this every time I start Rocksmith.