Hi Mary!
Thank you for bringing your question to the Disney Parks Moms Panel!
I am sorry you weren't able to select FastPass+ reservations for
Avatar Flight of Passage or
Na'vi River Journey. Those are both very new and popular, so they go quickly.
You can definitely keep trying to secure a FastPass+ for those two rides as your trip approaches. Sometimes, more times will open up closer to the date, either because other people cancel their times or because Disney opens more slots. Using the app is the easiest way to make them, even once inside the park. If they're not available on the app, then they are simply not available.
When it says "standby may be available" that just means that you can wait in the regular queue for the attraction. The reason it says "may be available" is because it's possible that the standby line would get long enough that they would close it to new Guests. This doesn't happen often, except when they close it down for the night.
I hope this helps! Please let me know if you have any other questions and enjoy
Pandora - The World of Avatar! ~Lori