Hi maria,
Thanks for contacting StyledThemes.
It seems you are using jetpack portfolio, and jetpack doesn’t provide any featured to edit permlink structure. And making changes on permalink will lead to fatal error-like page not found.
But still you can take help from this plugin, this may help you: https://wordpress.org/plugins/custom-post-type-permalinks/screenshots/
Let us know if you got any queries regarding to our theme, we will be more than glad to assist you.
Thanks
