Bibliography using latex on Overleaf website is not being passed

Question

I'm new to latex so forgive me if the answer's obvious. I'm trying to add references to my project, but the references just aren't appearing. It's simply blank where the references should be. I've tried lots of different ways of trying to insert the bibliography(including some I've found on stackeroverflow) but I just can't get it to work and I can't figure out what the issue is. As I said I've tried numerous methods of inserting the bibliography but for the sake of this post I'll only post a couple of them and perhaps one of you can spot the issue. I'm not sure if this is relevant but I'm writing this on Overleaf.

\documentclass{article}

\usepackage[utf8]{inputenc}
\usepackage[english]{babel}

\usepackage[letterpaper,top=2cm,bottom=2cm,left=3cm,right=3cm,marginparwidth=1.75cm]{geometry}

\usepackage{amsmath}
\usepackage{graphicx}
\usepackage[colorlinks=true, allcolors=blue]{hyperref}

\usepackage[
backend=biber,
style=alphabetic,
sorting=ynt
]{biblatex}
\addbibresource{bibliography.bib}

\title{}
\author{}

\begin{document}
\maketitle

\printbibliography
\end{document}

I've removed the main body of my document but everything of relevance should be there.

Here is the bibliography file:

@online{knuthwebsite,
    author = "Donald Knuth",
    title = "Knuth: Computers and Typesetting",
    url  = "http://www-cs-faculty.stanford.edu/~uno/abcde.html",
    addendum = "(accessed: 01.09.2016)",
    keywords = "latex,knuth"
}

Another method I've tried is:

\documentclass{article}

\usepackage[utf8]{inputenc}
\usepackage[english]{babel}

\usepackage[letterpaper,top=2cm,bottom=2cm,left=3cm,right=3cm,marginparwidth=1.75cm]{geometry}

\usepackage{amsmath}
\usepackage{graphicx}
\usepackage[colorlinks=true, allcolors=blue]{hyperref}

\title{}
\author{}

\begin{document}
\maketitle

\bibliographystyle{plain}
\bibliography{bibliography}

\end{document}

This method only print the word 'References', which leads me to believe it's an issue with the contents of my .bib file, but I copied the copy of my .bib file from an example on the official Overleaf website (just until I get it to work, then I will add my own) so the contents should be correct.

Any help would be greatly appreciated.