<body> should be placed in front of the first address and the <head> around <h1> needs to be removed (<head> is only for meta data at the start of the page)
<tel> and <email> aren’t valid HTML elements. It’s fine to just use <strong>.
The datetime attribute of <time> needs padding zeros for day an month. This would be correct: <time datetime="2017-05-02">.
You wrote title: instead of title= for the HTML and CSS <abbr> that’s why they are not showing the tooltip.
The quote at the end of the letter should be surrounded by <q>. This will automatically add quotes and tell the browser that this part is a quotation: <q>Be awesome to each other.</q>
I hope that helps. Feel free to ask questions if something isn’t clear.
The reason is that the whole footer also needs to be part of <body>. If you move </body> to line 96 the error goes away.
The main structure of an HTML file is:
<!DOCTYPE html>
<html>
<head>
<!-- meta data, page title, linked CSS files, etc. -->
</head>
<body>
<!-- All the page content including navigation, sidebars, -->
<!-- headings, paragraphs, lists, footer, images, etc. -->
</body>
</html>
All the tags have to be where the comments are above; either in <head> or in <body>.
Does that make it clearer?