Make sure you have a solid background contrasting the document you'll be scanning. If you're scanning a document paper in a white sheet of paper, you need a black or other dark-colored background so you can scan it as a document using the Camera app.
You can also check if the Scene Optimizer is enabled. You can check through open Camera > click Settings (gear icon) > check if the Scene Optimizer is toggled on. If your Document Scan is still not working, you can reset the camera settings of your device. Here is an article that shows how to reset the camera settings of your phone: https://www.samsung.com/ph/support/mobile-devices/how-to-troubleshoot-camera-app-errors/
Hope these fixes can help you with your issue. Thanks!
