The placket of the dress shirt, just below the top collar button, seems to bump out slightly, making a gap.
There are a couple of ways to solve the problem; you may review each of them and decide which suits you best.
For a start, you may want to wear a necktie. As the necktie pulls the collar band up tight to the neck, it will also pull the placket tight and reduce any bumping. If the issue is not resolved despite wearing a necktie, we suggest that you give the dress shirt a wash. Washing helps to relieve the tension on the threads and ease the collar band to achieve its target size.
Should the problem persists, you may want to check if your shoulder width is too narrow. A narrow shoulder width can cause the ends of the shoulders to pull your dress shirt out at the sides and bumps the placket out. If that is the cause of the problem, we suggest that you widen the shoulder width.
Lastly, if nothing you had tried seem to work, the collar type you had chosen might be the cause of the problem as it does not fit the shape of your neck and shoulders. Changing the collar type to an English Wide Spread should resolve the issue.