I have had a similar issue. I have a 2018 Cayman, 8000 miles now. Three times I have had "Lane Change Assist Unavailable". Not an error, no warning, just non-functioning. The first time it happened, the dealer couldn't get me a loaner for a while -- no big deal I could wait. Before my scheduled appointment, it self-corrected and all was fine.
The second time it went out I got it right in. They found no faults or errors; performed an update to the software and rebooted. It was fine for almost 1 month exactly. Then the same message.
The second time I took it in, I was going on vacation so they had it for a week. This time they found an internal fault causing an invalid yaw rate value. He tested the sensor, brackets, wiring, and calibration. The tech did not find any issues with those systems, so proceeded to remove the bumper and check the rest of the sensors. Nothing was out of the ordinary, and software was up-to-date.
After conferring with PTEC, they were instructed to replace the master LCA control unit. The dealer considered it and decided to replace both master and slave units since this is my second time in with the same issue. Reinstalled, recalibrated, and now back in my hands.
I've only had it back for a couple of days; fingers crossed that all is well. The technician's report (about a page and a half) is pretty interesting read.