The crease at the center of the main screen is shown when Galaxy Fold device is unfolded. Is this a device defect?

The main display of the Galaxy Fold is made with an advanced polymer layer and adhesive that’s flexible and tough enough to endure repeated folding. Also, Galaxy Fold features a breakthrough design, with a tablet-sized screen that folds to fit comfortably in one hand.

To ensure this new form factor experience was as intuitive as possible, Samsung invented the Infinity Flex Display—a first-of-its-kind display technology that opens and closes naturally, just like a book. 

You may notice the crease at the center of the main screen due to characteristic of new Polymer material but it is a normal characteristic of device designed for Infinity Flex Display. And it does not  affect the display’s quality, lifespan and performance and will not hinder the experience while using the device or viewing content.