This can happen when you have Portrait Orientation Lock on. Go to Control Center and tap the button shown below.
If this doesn't help close out the app, restart your phone and try again.
Add Portrait Full-Body Slates to your Audition
If you want to add Portrait full-body slates to your audition upgrade to SlatablePro. You can import up to 15 seconds of Portrait video. Go to SlatablePro Settings and import as many as you like. You can pre-Slate in different looks and then add them to your audition later, much like adding a headshot to your title cards.