Can't Add Unlimited Data option

I'm trying to add the unlimited data option to my plan but I'm constantly met with error code DFPLN4000 or it fails to load a page. I've read multiple support pages in here about needing to direct message support to get this figured out which means I have to complain here to be able to direct message them. Why the [Edited: Language] can't I add the unlimited data plan, I'm literally trying to pay you more money each month