Try

echo the_author_meta( 'nickname', $current_user->ID );

or you can also use;

$current_user = wp_get_current_user();
echo $current_user->nickname ;

Try

echo the_author_meta( 'nickname', $current_user->ID );

or you can also use;

$current_user = wp_get_current_user();
echo $current_user->nickname;

Try

<?php echo the_author_meta( 'nickname', $current_user->ID ); ?>

or you can also use;

<?php $current_user = wp_get_current_user();
echo $current_user->nickname ; ?> 

Try

echo the_author_meta( 'nickname', $current_user->ID );

or you can also use;

$current_user = wp_get_current_user();
echo $current_user->nickname ;