FYI: This is for MP server and the directory was added to addons. Nothing in the lua files show /sims so I'm guessing that's coming from one of the compiled files.

I recognize this may be far too late, but I had the same issue, so I'll contribute with my story for anyone with the same problem in the future.
I subscribed to, downloaded, and spawned the piano as normal. At first, it worked fine, with all models and functionality. However, on opening it on a different day, the seat was an error. This was in single player mode. It honestly wasn't that hard to fix in my case, as I simply closed and reopened gMod and uninstalled and reinstalled the mod, which got it working.