Fatal error: Call to undefined function wp_get_theme()

I am new to wordpress, I am trying to install a new plugin by uploading it to theme via my FTP program. when I do that and when I try to activate that theme in the wordpress i get this error:

Fatal error: Call to undefined function wp_get_theme()

I have seen few fixes such as rename it inside the theme folder among some other fixes none of them really help me, what can I do to get this problem fixed? I am using WordPress 3.2.1.

via Chebli Mohamed