The current_user_can() function, along with all the others in pluggable.php, are pluggable (hence the file name). This means they are explicitly defined after plugins are loaded so that plugins may override them (for example to implement a different authentication mechanism).
This means you cannot use any of the functions in pluggable.php directly in a plugin file (which you shouldn't do anyway). You must use them inside functions which are called on hooks.
For example:
function my_special_function() {
if ( current_user_can( 'do_whatever' ) )
// do your thing
}
add_action( 'plugins_loaded', 'my_special_function' );