Okay so I had this problem literally just then. I have a MacBook Pro (Touch Bar) - 2017, 13-inch. this issue occurred to me bc I uninstalled my Norton Security app as I had not renewed subscription. I noticed other people on this forum also did the same. I then had the ‘critical update required” message and was forced into an endless loop of inputing my internet password then it would take me to the same message again. from reading other suggestions, I rebooted in safe mode (simply turn off mac then turn back on and hold Left Shift till you see login screen), I was then going to follow the step to disconnect to internet then shut down but noticed in the internet ‘network preferences’ there was a Norton icon with a lock and simply pressed the ‘-‘ at the bottom left next to the ‘+’ which made the Norton icon disappear (I’m assuming deleted it) and then restarted my Mac normally and it was fine. I hope this suggestion works for someone out there!!