It turned out to be a permissions issue for me. I had followed the instructions to launch MSFS 2020, then FSUIP7, and finally Pilot2ATC, but I could not get a green connect light in P2ATC. I normally run P2ATC in administrator mode. I had done the same for FSUIP7, but that created a problem. I removed the admin attribute from FSUIPC7 and then it worked as advertised.