IntelliJ wrong source value when building

Question

I'm using IntelliJ IDEA Community Edition, but when I try to build I get:

        bullets.removeIf(Objects::isNull);
                                  ^
  (use -source 8 or higher to enable method references)

and it seems like source value 7 is being used:

According to this question, it's because of the project language level, but when I checked, everything was set to 15: Every module was also 15, too.

How do I change the source value?