Problem with PDF exporting and bibliography

Dear community,

I am pretty new to Jupyter notebook and rather fascinated by it. However, I have encountered a problem that I simply cannot manage to solve and I already spent quite some time on it. My problem is the following: I have enabled the latex environment for Jupyter notebook and would like to cite papers. Everything looks neat and works fine in the Jupyter notebook environment. However, as soon as I export the document to PDF, the citations do not properly work! In more detail, instead of the citation, I get a [?] indication.

I set up a minimal example on github about my problem: https://github.com/Elio-Rico/Index.git

You will find the jupyter notebook file, the pdf, and the bib tex file. If you have a look at the PDF, you will see that the citation as [?].

I appreciate any help of you and thanks in advance!