IntelliJ - Cannot resolve symbol after adding a label

 I added a label and came from :

doc.addError('vous ne pouvez pas ajouter de PJ');*

to :

doc.addError(System.Label.Document_messageErrSuppressionPJ);

It's fine on dev console, but IntelliJ isn't able to find the Custom Label ; I tried retrieving metadata but I'm missing something... Any idea how to resolve this ? I can delete my project and import it from scratch, but it's a waste of time even if it would do the work..

A screenshot from the error:

Thanks !

edit: my src file (that doesn't contain CustomLabels ?!)

edit2 : I'm trying to add custom labels by adding it to the package.xml