Yes, if currently you have all the plugins installed and activated, it makes sense to try import the demo content again. If it fails, please copy the error message and post it here.
And as for creating a Portfolio page manually, I can see that you’ve actually created a blog post type archive and put it on the menu under the Portfolio label. If you want to create a portfolio projects archive (like this one), please do this instead:
In your Admin Panel go to Pages > Add new. Set the Template of the page to “Portfolio Archive” (see screenshot). And publish the page.
Now this page will have all the projects you create in Admin Panel > Portfolio. And you can add it to the menu in a regular WP fashion.