When I login using Facebook, I get an error

It says: "com.facebook.sdk error 2" or “Unwanted User.”

Don't worry, you are of course a much wanted user! Sounds like you just need to give our app permission to use your Facebook authorization. This usually happens automatically, but sometimes settings can get inadvertently changed. To change that on your iPhone, go into Settings > Privacy > Facebook. Then find the 529 Garage in the list and tap the switch on to the right, so that it is green.