Why is the button to install a new module missing in the admin interface? I am new to Drupal and am attempting to install pathauto. But when I click on "extend," I don't have a "install new module" button.

Any suggestions?
You need to enable the core Update Manager module:


  Checks for available updates, and can securely install or update modules and themes via a web interface.


Emphasis mine.

Also make sure the user you're logging in as has the Administer software updates permission granted. If it's user 1, you already have access.