For a first game jam, it's surprisingly good ! I had a lot of fun playing your game !
Obviously, there are some things to improve, like the music (didn't loop) and sound effects (absent), or the fact that zombies can spawn on the player, but overall it's a good start.
There is also a bug when choosing where to place the wood or stone block : you can actually shoot zombies and being invincible.
I completely agree with quentin0133 and i think he said everything very well besides that. I would only highlight the fact that killing zombies is a bit pointless unfortunately, you can run in a direction and survive like that. Also one VERY important thing : set the "Is Kinematic" property of your rigidbody to true, because otherwise everything would move when the game is paused, and it also adds involontary movements to the enemies and to the player ...
That's a good work, keep it up !