There have been several posts about this error appearing when trying to download games through the itch launcher. All the posts I was able to find were from users aimed at users. I was having this issue with my own game and it took me longer than it should have to fix it since there was nobody on the development side talking about what caused the issue. I just wanted to share what solved the issue for me:

On your "Edit game" page, under "Uploads", check to see if you selected the correct filetypes/platforms for your files.

That's it. I forgot to do that when uploading and the error I was receiving didn't help solve the issue, so I hope this helps if anyone else has this problem!