I've seen this issue on here, but the difference with my situation is that I was able to upgrade through the app earlier today, but suddenly am unable to.
I'm in the midst of deciding on a new phone and was comparing prices and features just a couple hours ago. Now as I went back to check one last time before purchasing, under the "which line is this for?" the buttons for "upgrade" and "add a new line" are grayed out, I can't see the change in price as I cycle through iPhone storage options, and it tells me that I need to call the 1-800 number or go into a store. I hadn't seen this until now. I have tried both the app and logging in with a browser. Same issue.
It is currently 7:57pm central time. Does it remove the option to upgrade online after a certain hour? This is literally the only variable between now and a few hours ago, otherwise I have no idea what it could be.