Reply here if you have never been able to sign in to iOS or Android Waypoint.
This is from our ATLAS Support FAQ. Definitely give it a try.
Every time I sign into the app I get an error message, such as, “We couldn’t sign you in…”
If you haven’t accepted the latest Xbox LIVE Terms of Use on your Xbox 360 or on Xbox.com, you won’t be able to use Halo Waypoint. Sign into Xbox LIVE on your Xbox 360 or Xbox.com, agree to the Terms of Use, and then try signing into Halo Waypoint again.