Nick, looks like it cannot find the magnitudes of comparison stars in the ensemble. I’ve just went all the way – opening a 16-bit FITS file (ugc7513_000.fits), doing Iteratively Subtracted PSF Photometry, then set catalog to Gaia DR2, CCD Filter to G manually, ran Get Comparison Stars, then Find Regression Model and finally was able to get differential magnitudes by mouse-clicking on the sources. Try removing the old .phot file and executing the same steps. I’m going out of town for a couple of days but hopefully will be able to assist with delays.
Max