Configuration setting failed

I could install Documents Seller successfully.
However, when I try to set up a configuration firstly and put SAVE button after entering data,
it displayed Copy Failed and I couldn't change the setting of configuration.
I tried again and again and checked the permission of some files.
But I couldn't find the reason and how to configure it successfully.
How can I cope with this?

Hi

Can you make sure that you are using latest version of the extension(2.5.6) ? Could you send me FTP account and Joomla administrator account of the site so that I can check and fix it for you ?

Thanks ,

Tuan