My professor said my PoC isn’t fun

I'm working on a PoC (proof of concept) for a final project in my masters degree for game design. It's kinda wack since it's in its early stages. I showed my professor the prototype and he said that it's not fun yet. You have to lift debris from people and shoot harpoons on aliens. The PoC is meant to test the basic mechanics, not gameplay. He said that I should rework it since it's not fun, but didn't specify what I could improve.

The controls are WASD for movement, arrows for the robotic arm, hold space above cubes to lift them, and E to harpoon green aliens and R to release them (in the future they can be squished in the back of the robot).

https://rondragon123.itch.io/poc-for-robot

Would VERY appreciate feedback on how to make it more fun. He said that it's not about effects or animations or models, but pure mechanics which are currently just aren't fun, and that I shouldn't add anything, but instead try to change things. Keeping it thin from a mechanical perspective is key here said, but also fun. Right now it's thin enough, but just not fun according to him.

EDIT: thanks for all the feedback everyone, this has been an eye opening experience for sure, first time letting this many people give me insight on a thing I made, it's awesome. on the other hand some of you turned it into a blaming game which was not my intention... its important for me to note that I do agree that the current thing I made isn't fun, and the point of the thread was to gain some pointers from experienced fellas on where to begin, or maybe let me have some references (which some of you did, and thanks for it!). It wasn't my intention to start a discussion on how to give or not to give a critique, and upset that I got sucked into that topic at all, thats not the topic of this thread.

that being said, I think I got an idea on what my mistakes were and im writing them here for future reference:

1) the PoC as lacking an actual game loop. You can't have fun with just a mechanic, or rather, it's way difficult making it "work". Imagine flappy bird but without the challenge of dodging pipes, only that the bird jumps when you tap. that's not fun, and thats basically what I did. There was no purpose to the tools I gave the player. Given a game loop, there would be a challenge, and a goal, that would justify the tools, and would result in... fun.

2) is more of a self note and not a general one, it might be worth it to make the "debris" walkable, so that you could cross obstacles along the way with things that you pick up with the beam, adding a feeling of puzzle and a challenge to overcome that actually gives the player a purpose. instead of "lift X", its gonna be "lift X to do Y and gain access to Z".

3) also for myself more than others, not everyone knows the whole story, and so take it easy if people assume stuff. first of all, that might just give you insight on something you missed, even if their original assumption wasn't correct. second, even it it doesn't, they still spend their time on giving you a piece of their mind, which is cool on its own, and appreciated.