Allow coupon only for customers who already have placed orders.

functions.php

<?php

add_filter('woocommerce_coupon_is_valid', 'wcbr_coupon_is_valid', 10, 2);

function wcbr_coupon_is_valid( $valid, $coupon ) {
    global $woocommerce;

    $customer = $woocommerce->customer;

    if ( wc_get_customer_order_count( $customer->get_id() ) < 1 ) {
        return false;
    }

    return $valid;
}