This happens because you do not have permission to write in the htdocs folder. If I were you I would change the folder where to put Grav. In the Apache directory there should be these two folders, among other: sites-available and sites-enabled. The first is the 000-default.conf file, in the second a shortcut file to it. You should change the DocumentRoot directive with a path which you have write rights: for example, change /opt/lampp/htdocs/ in /home/<your-username>/htdocs. Same thing for the Directory directive. You must do this by Admin, so sudo -s, etc.