SketchUp viewer doesn’t open files on iPadpro3
If you have another application installed that can handle SKP files, that one may be taking the file and stopping SketchUp Viewer from seeing the file. You could either uninstall and reinstall the other app, then SketchUp would get the first try to open the file, or you could transfer the file to the Files app, and open the file in SketchUp Viewer.