Use Google

What are you trying to do? Why do you include the engine? Are you writing a script outside of the engine?

a) User is not an ElggObject

b) getDisplayName() is not a static method so you need to call it on a concrete instance of the class

c) Elgg has API to retrieve a logged in user

d) Always make sure there IS a logged in user, before trying to do anything with the entity

require_once '/path/to/vendor/autoload.php';

\Elgg\Application::start();

$user = elgg_get_logged_in_user_entity();
if ($user) {
    echo "The display name is {$user->getDisplayName()}";
} else {
    echo "User is not authenticated";
}

Thank you for the knowledge. I am new to elgg and learning it. How can use the API to retrieve logged in user data? I am just going through the documentation but not able to follow it up.

Thanks

Yash 

Look at the code I posted