Why I can't open photos or videos in Photos app on Macbook pro OS Catalina?

You need to import photos and videos to browse or edit them in Photos. Photos is a Photo manager, not just an editor. Import the photos into the photos Library, if you want to browse them with Photos.

On Catalina you can use the Finder as a photo browser. Just open a folder of photos in the Finder, switch the Finder window to Gallery View, by clicking the Gallery icon in the toolbar, then enable Full Screen mode. You can now step the photos with the arrow keys or by clicking the thumbnails in the film strip, for example:

And if you switch the Finder window to column view, the right column will show you some exif information for your photo:

If you are already inside the Photos app (as I think you are indicating), just double-click on a thumbnail (or hit space bar if an image is selected) to enlarge the image. Then you can use right and left keys to switch the focus to different images. Double-click again (or hit space bar) to return to the array of thumbnails.