I can reproduce your issue with Nautilus 3.26.4 on Ubuntu 18.04 when using the Ambiance gtk3 theme which is the default in Ubuntu 18.04. Pressing Tab any number of times does not apparently get me to the pane listing files and folders.
But, if you switch to the High Contrast gtk3 theme, which though not aesthetic, makes it very clear when you've tabbed to the point that the file/folder pane is active. (To change gtk themes you'll need to install
gnome-tweaks from the repos.)
In the following animation, I've used the materia-gtk-theme from the repos to show that just tabbing is enough to get to the pane containing the list of files/folders. (I converted images to grayscale so that the animated gif falls below the site's size limit of 2MB.)
First, repeatedly pressing Tab works down the left pane, aka Places, and then enters the pane with the file/folders list. Further tabbing moves to the area above the file list pane.
In any case, if you like using the keyboard primarily, why not try something like ranger or nnn? There are many nice YouTube videos on ranger and other keyboard-driven file managers.