If your name was changed automatically upon logging in for the first time, that means the name from your platform isn't acceptable to the filter. You still have one name change remaining, as is shown by the notification when you attempt to do so saying there's only one. When going to change your name, please note that you cannot use the name that has been filtered, even if the Suggestions suggest such a name. The Suggestions just pull from your platform names, and as you were Guardian'd automatically, that means it's unacceptable to use as a BungieName, so you'll have to use something else. You'll need to choose something else other than the name you're trying to use. Please note that names that break the CoC aren't acceptable, even if the filter doesn't catch them, and if you choose one that breaks one of the Code of Conduct rules, you'll end up with a permanent Guardian name, and then you really won't be able to change it.