I have Xplane 11 on one PC and Xplane12 on a second PC.
Simbrief creates a flight plan KDTWKATL01.flp in my download folder for Flight Factor 767 (v1.6.12) and I copy it to :
D:\X-Plane 11\Aircraft\Boeing767-300ER\co-routes
The route is not found using the FMC.
For Xplane12 the route is found OK.
Edit:
I found my problem.
The Simbrief route needed to be copied to two CoRoute folders because of an EFIS (FPDS) update.