What does the "return"??

Returns are the sale returns, please check the documentation.pdf in your download. Thank you

What do the promos under the setting?

That's buy one and get one free. You should try on live demo to test and if don't understand let me know.

I still not understand about promos.

Can you make the demo?

What I am trying to say is that you need to try it first. Once you have added the promo and sold that product, you will get the idea. Please refer to attachment.

But the watermelon still is 100 quantity. As you say, If I buy 1 Minion Banas, the watermelon still is 99 quantity

I am not sure how are you doing. Please let me know the steps so that I can check.

The correct way would be add promo, then to sell the item and see if the free item is auto added and then check back the quantity for both items. If you are testing on demo, please keep in mind demo is being reset every hours.

Can I post the video?