Hi Jocelyn,
Not sure how we can help with the first part. Could you provide a little more detail on what you want to achieve? Hopefully someone in the foruns will be able to point you in the right direction.
Regarding the error you are getting, it sounds like the Crop you are defining is larger than the image of your camera. In order to fix it, you should reset the ROI. In the top right corner of Bonsai window, under Properties, input '0,0,0,0' in the RegionOfInterest property. This will reset the ROI, and your workflow should be able to run again. If you want to draw a new crop, click in the "..." icon available under the same property and draw the desired rectangular ROI.
Hope this helps,
Bruno