Entering 6 digit verification -> "This email address is not available. Choose a different address." !?

Recently I received that someone might've logged in using my Apple ID.

So I followed the instruction of changing the password.

1. changed password

2. Apple ID account login page

3. security question I forgot, so reset security question

4. Apple ID account login page again

5. security question (set above 4.) answered correctly

6. verify email address modal

7. verification code modal

8. check my mail box, entered verification code

9. popup "This email address is not available. Choose a different address."

It does not make sense. Any help, much appreciated.

screenshot for the step 9