Hello,
Ok, so if i understand you are on the automatic discount page, you selected new and now you want to choose a coupon code. To do this, start typing the coupon code, and after 2 letters you will see a dropdown. Select it from the dropdown. It has to be selected otherwise you will receive the error. You should then be able to save.
Here is an example of what the dropdown looks like:
https://awodev.com/images/v1/blogs/6/auto-edit.png