None of this worked for me. So, I hit Deny and opened the Key Access app. I found the Apple Developer passkey and double clicked it. Clicked Access Control tab and hit "Allow all applications to access this item." It asked for a login again, and it WAS NOT the Apple Developer password that worked--it was my laptop code. Reran the Flutter app -- Apple Developer password login popped up again, and I entered my laptop password again. Flutter app running.