Don't give up. If the site is saying that there is nothing at all for all the dates of your stay, that probably means that those dates aren't open to you yet for whatever reason. Even with the invitation, there seems to be no logic to when times open up, and apparently it's different for different people. Some people can make FP+ a month or more in advance, and others seem to have to wait til within a week or so of their trip. Keep checking the site. When your dates do open up, you'll probably have a choice of times for each day. The BoG FP+ process just isn't that well known to the average WDW visitor, so there are nearly alwaye times available as long as your dates have opened up. And the WDW dining people don't have a clue about BoG FP+, since it's totally separate from the regular ADRs that show up in your
MDE. Almost everything that's been reportedly said by CMs about the BoG FP+ has been wrong, according to the real world experience of the posters here.