But probably the reason why you get 0.000 pix error is that you have created the marker using Add Marker option either on the single image in Photo view mode, or in the Model view mode.
I suggest also to adjust the projections of the marker on other images, so that you get "green flags" indicating the marker's projections on all the relevant photos and you will likely get non-zero value for the reprojection error. While currently, if you use the described approach with Add Marker, the projections of the marker fully correspond to the location of its 3D position reprojected back on the images.