Cessna Nighthawk Garmin 1000 AP does not follow Simbrief Flight Plan

I have just flown my first flight using a Simbrief Flight Plan from KBFI to KTIW.
I expected that the AP would follow the plan, but that did not happen. I had to “hand steer” the Cessna back on course.
What am I doing wrong?

You were in ROL mode in the AP. It’s true, it didnt engage. It shouldnt have engaged either. You may want to watch a video or two on the Garmin 1000 autopilot

I have…and here is my flight using a Simbrief flight plan…