Hello,
I've faced the same issue and the bizzare thing was the I'm the owner of the developer account. Yesterday I could access the itunes connect without issues but today I was getting an "Your Apple ID isn't enabled for iTunes Connect." message.
I've discovered that using the other URL for the developer program worked for me (Not sure if Apple changed something on their end or not). However this is the link that worked for me (only for developers):
https://appstoreconnect.apple.com/login
Don't use the "iTunesConnect" one that OP used.
Alternatively go to Google and type "IDMSA APPLE" and click the first link and you can login from there too.
Hope that helps anyone.
Topic:
App Store Distribution & Marketing
SubTopic:
App Store Connect
Tags: