If you signed up with a social network account, the connection is based on whichever account is currently logged in to that social network. If a different account is logged in than the one you used to sign up, you will be taken to a different CLASS101 account. Please verify your account on that service and try again.
Kakao login requires that you are already logged in to KakaoTalk on your device with the same account first.
If you're not sure which account you signed up with, go to the Find Account option on the login screen and enter the phone number you used when signing up. Even if your number has changed, you can still look it up using your old number.
⚠️ Even with the same email address, Kakao, Naver, and email sign-ups are each separate accounts and cannot be merged.