Please make sure that the portfolio slug is not the same as any page slug.
For example, the theme by default uses "portfolio" as a portfolio slug. If you also have a page that has the same slug as "portfolio", please change the page slug to something else (eg. "my-portfolio").
OR, you might want to change the portfolio slug instead. To do that, go to "Appearance > Customize > Portfolio Options" and scroll down to the bottom. You will see the options to change portfolio slugs there.
After that, go to "Settings > Permalinks" and click save.
This should help.