Drupal 8 Doesn't Have Install Module Button
On a fresh install of Drupal 8, or an install with a theme that you need to import a SQL script for, you may find that the option to install a new module is not visible in /admin/modules, nor can you install a new module by hitting the URL directly on /admin/modules/install
To resolve this you'll need to enable the update manager. Scroll down to the Update Manager option (as seen below) and check the checkbox.
Then scroll down and click install.
Now you will be able to install new modules through the web interface as per the below. Using Drush or Composer is the more "Drupal" way to do things, but sometimes it's nice / easy to install through a few clicks.
Why this isn't enabled by default is anyone's guess, but by enabling from here, you'll be able to then install further modules through the GUI. To be fair to Drupal, it does state at the top of the page that you need to enable this, but if you've found your way here - well, you missed that line just like I did.