Create your main portfolio page and assign it properly so your portfolio items will populate this page automatically.
Create a portfolio page
Create your portfolio page, name it any way you want, and publish it.
Items inside the main portfolio page
Please leave the main portfolio page untouched. Do not add any content, items, or shortcodes. The main portfolio page should populate with posts by itself.
Assign the portfolio page
On this page, you will notice a dropdown menu labeled as Portfolio Page with all of the pages on your website.
Select a previously created page there. It will then populate with your single portfolio pages.