Unable to Sign In to iTunes Connect: "Your Apple ID isn't enabled for iTunes Connect"
The solution is simple: Login with this account to iTunes (not iTunesConnect). I am sure you didn't do that. You can take your iPhone or iPad to do that or download the iTunes application.
It is not enough to be a member of the Apple Developer Account / Member Center team.
The account must also be added as an iTunes Connect User.
From an existing iTunes Connect admin account, add the new user to iTunes Connect:
iTunes Connect > Users and Roles > "+" > Add iTunes Connect User
