I tested you code and it looks good. I would imagine is has to do with your server setup.
This line looks like it could be problematic
<link rel="shortcut icon" href="<?php echo get_stylesheet_directory_uri(); ?>/images/favicon.ico" />
Do you have php enabled on your server? Does your file have a php extension?
My thought would be that the ?>
might be ending your link tag and causing some sort of issue.
If you have a url where I could look at it I might be able to help you out more.
EDIT:
the url you sent does what I am describing above.
<DOCTYPE html>
<html>
<head>
<title> YHS Alumni Association | Home </title>
<link rel="stylesheet" type="text/css" href="style.css">
<meta charset="utf-8">
<link rel="shortcut icon" href="Fatal error: Call to undefined function get_stylesheet_directory_uri() in /home/u190182631/public_html/YMHS Alumni Association Website/home.html on line 7
Remove the line <link rel="shortcut icon" href="<?php echo get_stylesheet_directory_uri(); ?>/images/favicon.ico" />
and it should work.
The php function you want to call does not exist.