Query Column of Specific ID from Database Table

You have 2 two problems here:
– You can’t use $wpdb-> with custom tables because [postmeta, posts, …] are variables in the class $wpdb.
– WHERE ‘id’= 2 is not correct WHERE id = 2

So it should work:

global $wpdb;
$table_name = "_customtable";
$retrieve_data = $wpdb->get_results( "SELECT * FROM $table_name WHERE id = 2" );
foreach ($retrieve_data as $retrieved_data) {
    echo $retrieved_data->item_value;  }

Regards.