The most common cause is using a different sign-in method than the one you originally used.
If you signed up with Apple
Tap Continue with Apple and use the same Apple ID you registered with.
Note: If you used Apple's Hide My Email feature when signing up, your account is linked to the relay email Apple generated — not your personal email. Use Continue with Apple to access it.
If you signed up with Google
Tap Continue with Google and select the correct account.
If you signed up with email
Enter your exact email address. Tap Forgot Password if needed.
Still can't log in?
Contact support through the Help Form and include your account email or phone number.