APA citing gives bold display and error

Question

I've been searching this site over and over to find a solution to my problem, but I can't find it and what I've found does not seem to work.

My problem is as follows: I'm trying to cite APA style in my ShareLaTeX file, but whenever I type \cite{} and select one of the proposed writers from my .bib file, an error occurs. The citation appears in bold, and the error is:

Citation 'Slagter2012' on page 1 undefined on input line 24.:
You have cited something which is not included in your bibliography.
Make sure that the citation (\cite{...}) has a corresponding key
in your bibliography, and that both are spelled the same way.

Now I've checked this over and over and it seems all the same to me. Is here something wrong in my file?

\documentclass{article}    
\usepackage[utf8]{inputenc}    
\usepackage[affil-it]{authblk}    
\usepackage[american]{babel}    
\usepackage{csquotes}    
\usepackage[style=apa]{biblatex}    
\DeclareLanguageMapping{american}{american-apa}
\addbibresource{article.bib}    
\title{MyArticle}    
\author{MyName}    
\date{Dated: \today}    
\affil{University of Amsterdam}    
\begin{document}    
\maketitle    
\section{Introduction}    
This is the introduction.\cite{Slagter2012}    
\printbibliography
\end{document}

This is my .bib file

@article{Slagter2012,
author = {Slagter, Heleen A. and Tomer, Rachel and Christian, Bradley T. 
    and Fox, Andrew S. and Colzato, Lorenza S. and King, Carlye R. 
    and Murali, Dhanabalan and Davidson, Richard J.},
isbn = {1530-8898 (Electronic)$\backslash$n0898-929X (Linking)},
issn = {0898-929X},
journal = {Journal of Cognitive Neuroscience},
number = {9},
pages = {1932--1940},
pmid = {22663253},
title = {{PET Evidence for a Role for Striatal Dopamine in the 
    Attentional Blink: Functional Implications}},
volume = {24},
year = {2012}
}

And this is the output

What am I doing wrong?

NOTE I am completely new to this, be as clear as possible otherwise I will not understand ;)