I would suggest:
Make sure you have the latest version of Walkout Songs App 2.5.28
Kill and restart the Apple Music app as that is the host application for the iTunes song picker.
Kill and restart the Walkout Song app
Go to your device settings app and make sure the Walkout Song app has permission to access your "Media & Apple Music"
Power down and restart your device
If none of that works... do a screen recording of you trying to open the iTunes song picker and email it to me at walkoutsongdj @ gmail.com