This usually happens for one of two reasons:
You used Apple's Hide My Email. Apple shares a private relay email instead of your real one, so it doesn't match your existing account. Oobit asks you to confirm before creating a new account this way.
You signed in with a different email from the one on your Oobit account.
To fix it: sign in with your original method (email and password), then go to Settings and link Apple or Google there. From then on, Apple or Google will open your original account.
If you see "account already exists", there's already an Oobit account on that email. Sign in with your password first, then link Apple or Google in Settings. Oobit never merges accounts automatically.