Unable to Sign In to iTunes Connect: "Your Apple ID isn't enabled for iTunes Connect"

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

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.

If your Apple Id is added to your team account recently, you need to activate it first by clicking on the link that Apple sent to you with the activation code. After that, you should be able to login to iTunes connect.
P/S: You may need to check your internet connection, firewall, proxy setting to make sure you can connect to itunes connect server. This happened to me as I used my company network to login. After switched to my 3G hotspot, it worked like a charm.