If you're trying to log in to 1SE using your Apple ID but are getting an error message, here's how to fix it:
Open the Settings app, then tap [your name].
Tap Password & Security.
Tap Apps Using Your Apple ID.
Select 1 Second Everyday: Video Diary from the list.
Tap Stop Using Apple ID at the bottom of the screen.
Tap Stop Using in the alert shown.
Open 1SE, then open the Profile tab.
Tap the Sign In or Create Account button.
Select Continue with Apple and follow the sign-in process.
For additional information, here's a helpful article by Apple on this feature: https://support.apple.com/en-us/HT210426.