Fix on wording

[phackwer]

Please, change this subscription process to have a better worrding. We've got 2 seniors and a CEO stuck tryig to understand why, even after creating account and subscription with Credit Card details, they couldn't use AppCenter. The reason is simple: the message should say "This subscription isn't allowed for billing BECAUSE YOU NEED TO UPGRADE FIRST TO PAY-AS-YOU-GO". We searched everywhere and all internet to find the problem and no one online had this answer.

It's not a bad process if you have all the words, but right now it's impossible to have the words!