How can I edit a submission's coupon code after it has already been submitted?

Hello,

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

The highlighted area doesn't let me add anything.

First of all, please accept my apologies for any inconvenience.

Could you please provide us with more information such as the form ID or URL and some submission details that allow us to pinpoint the submission you're having issues with, so we can inspect and assist you better?

Thank you in advance. We are awaiting your reply.