New line

Have you missed any bills in the past? If not, try placing the order with customer service or in store.

I have not. I even paid off one phone to then try to add the fourth line but it won’t let me. I did it in store. I’m trying to get a $5.99 installment plan that AT&T is offering but it’s saying I have to pay for the phone in full upfront.

The online website/app for ordering is known for having glitches like this more than often unfortunately… it’s very rare to have a down payment when adding a line especially when you regularly pay your bills, no matter your credit. Going in store with your ID is your best bet, they can ship whatever phone as well.

Thanks. Everything I’ve done so far has been in store. 

I just like any other credit, there is a maximum amount that you have been approved for through AT&T's super secret internal credit score. Since you already have two phones on installments, you may have already maxed out the amount that AT&T is willing to allow. Even if you are receiving bill credits which over 36 months will make those phones free or deeply discounted, They still consider the full installment value of their phones your credit amount and your credit limit. 

Usually if you deal with people in the store, they will accept a large down payment on a phone. If you paid everything but $36, the net would eventually be $1 a month over 36 months.  Make sure you are dealing with a corporate store. Authorized retailers can't always do something like that