Ah! I was doing the wrong thing! I downloaded the file itself first! (V1.04_Raft_MacUniversal.zip)
All I had to do was what you said TR1BE2!
I deleted the zip file first, went to the Itch homepage and downloaded the Itch app,
THEN, downloaded the Raft through the Itch app!
Voila! I successfully launched the game!!! Thank you~!!!