To create a global Portfolio page that will show all your portfolio posts go to your dashboard Pages tab>Add new. Type a page title and select Portfolio template in Page attributes

Click Publish button and see your portfolio

If you need some content shown before portfolio items, then add it into Text/Visual editor or use Elementor page builder

Now you can work over portfolio design and options.

Featured image

Set featured image that will be shown on post page 

Sidebar Picker

Select desired sidebar area.