Tesco grocery app driving me insane, how do I add a coupon?

[Nifedipine]

Hello! I am going around in circles with the bloody Tesco grocery app. Have an existing order, which I am amending, and would like to add a coupon but for the life of me cannot figure how!
When I press save changes, it flashes up with a circle going around and in the background I see an 'add coupon' option, but there is no way for me to click on that, it just checks out and that's it. This is extremely annoying and I have now spent crazy time trying to figure it! Any help much appreciated, thank you!

I would just use the online browser website to add coupons because the app does have a habit of just checking out even if amending an order.

I agree with H1978, it's easier to use the website than app to add a coupon/voucher if you've already checked out.