I have always had an issue logging into YouTube from my iPhone to upload video from my camera roll. No matter how many times I tried, I would get an error that it was unable to authenticate.
I finally realized that the issue was my two-step verification with my Google account. The YouTube login on phones doesn’t allow for the full process to run, so it was just bouncing back as invalid.
My fix was to go into my Google account settings and create an application specific password for YouTube on iPhone and use that password. Worked perfectly and solved a huge headache!