I have no explanation for that image. But please do follow my suggestions above: You did not subscribe to the NeuroAnatomy update site: I know this because the menu structure to start SNT changes slightly from Plugins>Segmentation>Simple Neurite Tracer… to Plugins>NeuroAnatomy>SNT… after subscription. I also don’t think you followed the instructions at https://imagej.net/SNT:_Step-By-Step_Instructions#Tracing_in_the_Legacy_3D_Viewer: You shouldn’t run Plugins > 3D Viewer AND instantiate a 3D viewer from within SNT at the same time. That is likely to create conflicts between the two instances of the 3D Viewer.
At this point I strongly you suggest to start afresh, by downloading a new Fiji installation (you can rename it something like Fiji_SNT_Debug.app, and follow the instructions above. My guess is that the problem will go away, and once you have it working, we can always troubleshoot what is going wrong on your current installation.