It is not possible to upload a file to the root directory of your site and Norton requires you to do this in order to verify ownership.
However, it does seem like they offer an alternative that will work with Yola. See this post from their Community: Help with Norton Safe Verification.
From this post, it seems that you need to send them a support request explaining that you are unable to upload a file to your root directory and that you need to use the metatag verification method instead.
They then suggest adding it as a keyword metatag. You can do this by going to Page > Metadata and pasting it in the designated keyword box.