After you activated the theme; sometimes you need to activate Elementor for portfolio pages.
So please follow these steps below;
- Navigate "Elementor > Settings" on your admin dashboard.
- You'll see "Post Types" section below "General" tab.
- Click on "Portfolio" and save changes.
After that; in portfolio pages there'll be a button appears "Edit with Elementor" just click on it and edit/customize the portfolio page.