I can't change my password or phone number

 
When access this website I've logged in by Gmail. I interest in selling my expert advisor, after clicking seller the website introduced to put phone number. But before click ok they ask my current password. I tries many times to put my email password because I don't know what is my current password ? as I logged in via Gmail. After that I tried to recover and change my password, Finally your system give me a new password and I used that password to login. But there is notification that "this e-mail cannot be used" I've tried to change other email for logging in all results are similar with the situation above. How or what should I do, why the system so complicate and difficult to understand like this. 
 
Chatmongkon Kaewrueandee:
I've tried to change other email for logging in

Your login for this website (and for Community tab of Metatrader) is chatmongkonkeaw because the link to your profile is
https://www.mql5.com/en/users/chatmongkonkeaw
Do not use your email for login.

Reason: