I used this code to do citations for the following reference
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\usepackage[backend=biber,style=authoryear]{biblatex}
\addbibresource{SMC_paper.bib}
\begin{document}
Let's cite! The Einstein's journal paper \cite{al2004sliding}
\printbibliography
\end{document}
this reference:
@article{al2004sliding,
title = {Sliding mode control of a magnetic levitation system},
author = {Al-Muthairi, NF and Zribi, M},
journal = {Mathematical Problems in Engineering},
volume = {2004},
number = {2},
pages = {93--107},
year = {2004},
publisher = {Hindawi Publishing Corporation}
}
but it gives me this result
Let's cite! The Einstein's journal paper al2004sliding
.logand.blgfiles. If you can't find a.blgfile, it is very likely you didn't run Biber, see Question mark instead of citation number and Biblatex with Biber: Configuring my editor to avoid undefined citations. If your.blgfile mentions BibTeX and not Biber you need to make sure to run Biber and not BibTeX. – moewe Sep 28 '15 at 15:23