Can't use free trial Sophos Central.

Hi taweephol 1976

Thank you for your query, as it says the email is already in use, so have you tried re-setting the password of that account ? And then check once you are able to access your account...
cloud.sophos.com

Yeah, I have reset the password but alert message Authentication failed.

In this case, would suggest contact the customer care team once, they will assist you further and help you recover your account. 

i can contact with email support@sophos.co.th. right? or should another contact.

please recommend contact for me. 

i can contact with email support@sophos.co.th. right? or should another contact.

please recommend contact for me. 

Here are the contact details according to your region, you may select: 
https://www.sophos.com/en-us/company/contact/apac

or also raise a service request here: https://support.sophos.com/support/s/?language=en_US