Hi Sergi,
If you used the Page Attributes to set the Page Template to Layout - Portfolio you will not be able to use the Page Builder as it is a specific page template for portfolio and not part of normal page view to be able to use the Page Builder.
Kindly click here for more information about the portfolio setup and here for the settings.
You either need to customize the Portfolio Page Template by customizing the theme using a Child Theme and Best Practices or you can use alternative methods to show the portfolio items inside a normal page that you can access to Page Builder.
You can use the Essential Grid plugin and set the source of the plugin to show portfolios and then use the Classic Essential Grid element to show the grid using the Page Builder.
Thank you.