Cannot even build a basic project

I have no idea what I am doing wrong.
I am currently using Ubuntu 18.04 bionic 5.12.
I have all the build tools installed : gcc, g++, make, cmake etc etc
But the button to build a new project is not enabled.
There is not even a way to tell JUCE where to find the binary (there’s only stuff for Android)
The tutorial given assumed everything will work out of the box.
Am I missing something?

In the folder Builds/LinuxMakefile you find the Makefile, so just type

cd Builds/LinuxMakefile
make -j 4

I think because there is no default IDE, the button is greyed out.

Any way to fix this? Like you said, how to set a default IDE?

There’s no “fix” because you can’t - Linux operating systems don’t have a default IDE. Projucer isn’t designed to have you associate any particular IDE either - its goal is fundamentally to generate projects files and manage projects in a genericised cross-platform way.

All (?) IDEs on Linux support Makefiles, so that’s the common point for you to start development.