Custom Permalinks Issue

I am trying to follow the instructions to set up permalinks here: https://www.visualportfolio.com/docs/settings/permalinks/#project-permalinks

I would like to change the word “portfolio” to “films” however, in following these instructions I can only get it to work if I also throw the %portfolio_category% the %portfolio_page_slug%/ in the Custom Base option. If I just try to do “films/” if II get the error “A structure tag is required when using custom permalinks.” If I add %postname% and do “films/%postname%” in Custom base I get
a 404 error because the website puts %postname% in the url.

Is this just how it has to be (including category) to replace the word “portfolio”?

The page I need help with: [log in to see the link]