Hey,
So I finally figured this out. Your repository was on an older group of repositories which was using the old toolbar V1 and the @nuxt/prismic module previews only work with toolbar V2.
You can tell if your repository is using V2 of the toolbar if the script it your repository preview settings includes ?new=true like below.
<script type="text/javascript" src="https://static.cdn.prismic.io/prismic.min.js?new=true"></script>
You can find these settings at https://your-repo-name.prismic.io/settings/previews/
I moved your repository to the correct group for you so this should work now 
For any users in the future with this issue you can contact the activations team as described in the following thread and ask for us to activate V2 of the toolbar for you.
Thanks.