Try these steps in order, and check the app after each one:
Update the Oobit app to the latest version from the App Store or Google Play.
Update your phone's operating system.
Close the app fully and restart your phone.
Check your connection. Switch between Wi-Fi and mobile data, and turn off any VPN.
Clear the app cache (Android) or offload the app (iPhone: Settings, General, iPhone Storage, Oobit).
Reinstall the app. Your balance and card are stored on your account, not on your phone, so nothing is lost. Just sign back in.
If it still crashes, contact support through the app and tell us your phone model, operating system version and app version. A screen recording of the crash helps too.