Hi,
I have two websites in two separates my_webapp folders ( a blog and a Minigalnano photo website).
I upload my Hugo site using the rsync deploy functionality: Deployment with Rsync | Hugo which is very handy.
However I also sometimes upload using Filezilla.
You should only upload the files that are in the “public” folder of your generated Hugo website.
It should contain an index.html file.
So, I suggest you to first delete all the files in your www folder in Yunohost, then upload the files in the “public” folder to the www folder.