Solved by adding a package to my library, which used a default package.
I have a school project to build a Jar library containing a basic card management system, then use it in the main project (a blackjack game)
With IDEA I separated the modules, then I built the jar artifact. I now have all my classes in my
unresolved symbol Carte
It looks like the classes in your library are in the default package, i.e. they don't have a specified package name.
Carte is the name of the module, but not a Java package. I marked what I mean on this screenshot.
Java doesn't allow importing classes from the default package, as per Java language specification:
It is a compile time error to import a type from the unnamed package.
So, you should avoid the default package to be able to use the classes from a library.
Another possible reason of why the classes seem to be in the default package is that you've improperly packed the jar.