Can't order online

Why can't I order online? It's says after I click on add new line it says Sorry you can't place an order at this time, call customer support. Doing that will charge support fees right? I'm trying to change from Xfinity to T-Mobile since I have Internet with T-Mobile, but if its this difficult I might as well stay with Xfinity. I don't won't to call or go to the store when I can do it myself in 20mins instead 2 hours talking to a rep. Plus the online deal. This is ridiculous.