SFTP remote open permission denied

May be the user in question doesn't have write permission to public_html directory.

you could try this:

make sure your user is a member of the www-data group:

sudo usermod -a -G www-data $USER

then give group permissions:

sudo chmod -R 775 /var/www

Check this out -> https://www.linode.com/community/questions/11090/want-to-add-a-user-to-upload-files-using-filezilla-on-debian-server

Regards

The link in the above answer is now located at its new Community site home:

https://www.linode.com/community/questions/11090/want-to-add-a-user-to-upload-files-using-filezilla-on-debian-server