The same thing happened to me, I changed 2 things and the problem is gone (plugin version 3.2.5, WP version 4.9.8):
– the plugin created the database table with a wrong name – $table (this is the PHP variable in the code) – I changed it to {$wpdb->prefix}woocommerce_gift_coupon, where {$wpdb->prefix} is your prefix from the WP installlation
– if I bought a coupon as a guest, a new line wasn’t inserted into this table (so I was able to generate infinite coupons for every guest order) – I removed the key from the id_user column to allow value 0 for guests and the plugin works as expected.
