Drupal 8.x

I successfully got Drupal 8 installed on my Ubuntu 16.04 LTS machine, however I cannot access any other page other than the index / home (http://webserver/) page. When I try to manage the site (eg. http://webserver/admin/structure) I get a 404 error page like this:

Not Found

The requested URL /admin/structure was not found on this server.
Apache/2.4.18 (Ubuntu) Server at webserver Port 80

How can I resolve this issue?

In the "Best Practice Site Architecture in Drupal 8" video by Pantheon , one of the guys says that the use of taxonomy is more of a Drupal 6 design pattern, and that they barely use it anymore. Watch the video staring from here to hear for yourself what he says about this.

Which design patterns are recommended for Drupal 8 projects, that replaces the need for taxonomies? 

When I went to clear my cache on my Drupal 8.5.3 site using Drush, I got the above-referenced error message. I was able to clear the cache through the menu system, but this slows me down.

After doing some research, it appears that this is a symphony problem with a simple solution of changing the yaml files to UTF-8. So, I went into Notepad++ and opened my *.info.yml and *.libraries.yml to see if they were encoded in a different format. Nope, they were both UTF-8.

Has anyone else encountered this error and/or can someone lead me in the right direction?

Steve

When editing an added node, I can't figure out how to ensure the taxonomy field within my custom plugin populates with what was previously selected, using the "default_value." 

A second problem is that, when viewing the node, it shows the taxonomy term id -- not the term itself.

Here's what I've got:

   The site was Drupal 5.3.3, now update to 8.5.3, it finish the update, but can not open the site, it instantly show this error message from home page

    I can not find the answer anyway, please share your experience, thanks.