How can I edit a submission coupon code after it's completed?

I'm trying to edit/input a coupon code to a completed submission but it's not letting me click on it. 

I'm trying to click on the highlighted box but it doesn't let me add anything. 

I'm sorry, but you cannot. This option is automatically disabled on edit submission. Our payment API cannot handle edits on payment transactions yet. You'll notice that the product items are also disabled.

If you want, I can also send a request ticket about it through this thread. Let me know. Note that we cannot give any estimate to every ticket we made. We can only assure you that we'll be sending updates if available.