I purchased the TMB 900 in August 2020 and have a valid license.
I have (and always had) 3 monitors, but I'm using only one of them for the X-Plane. The layout of the monitors is in the attached image. My main monitor is monitor no. 3, and I use only monitor no. 1 for the X-Plane.
After reading your answer I suspected that GIZMO is trying to display its windows on the main monitor (no. 3). So, in while in X-Plane I activated the no. 3 monitor together with no. 1 monitor - and sure enough all the GIZMO windows were displayed there. So I was able to complete the registration.
The bottom line for me: this was very annoying experience that wasted me a lot of time! I couldn't even imagine that GIZMO will display its windows on a monitor not activated in the X-Plane.
Is it a bug? Can it be fixed?
Anyway, thanks for your reply - and your SUPERB TBM 900. It's a joy flying it!