Im not sure this will work for everyone, but I recently switched phones and think I left my apple ID on the old one I gave to my son, so I got the exact same error when opening "Find My", however all I did was go into settings, type "Find My" in the search bar. When I got to the find my section, I stopped sharing my location with my family first, then turned off "find my iphone" I restarted, then went back into find my and turned it back on, then shared with my family again and voila! Hope it helps, apologies if it doesnt.