WordPress plugin blog creation

pluginprefix_setup_post_type() is a method inside the SoaneNews class, not a function, but inside your activate() method you’ve called a function with that name:

function activate(){
    pluginprefix_setup_post_type();
    flush_rewrite_rules();
}

To call the pluginprefix_setup_post_type() method from within the class, you need to do it like this:

$this->pluginprefix_setup_post_type();