Help with missing output from cubegen utility after density calculation in Gaussian16
You aren't getting the density data in your cube file because you haven't specified the kind keyword in your cubegen command. Gaussian allows a variety of different types of cube files to be generated, including electronic densities/spin-densities/gradients, MOs, and electrostatic potential, along with magnetic current and shielding densities. Without specifying the type of cube file you want, the program doesn't know what to generate.

The reason it can't determine what you want by parsing the chk file is that the file in general will contain information that could be used for multiple types of cubes. The program could hypothetically use the first valid piece of information or some specific type of cube data by default, but this would often produce an undesired output. It could also make all valid cubes files, but these take up a lot of memory so this would be wasteful. They presumably decided the default case should be to do nothing if a type of cube file isn't specified so as not to produce unnecessary or incorrect output.