I just checked and your theme is showing up as a “Broken” theme. Go to Appearance > Themes and you can also go to Appearance > Editor and you’ll see the notice at the top.
If you’ve made edits to your files and there’s an error it can cause this to happen. Do you remember which files you’ve made edits to?
There is supposed to be a Template dropdown on the right side of the Page editor to allow you to select the Portfolio template.
You can first try deactivating and reactivating the theme to see if that fixes the issue. If that doesn’t work then you’ll have to tell me what all changes you made to your files (not the CSS file though, as that won’t affect it).
Just let me know
Edit: Also, are you logged in as the Administrator? I see you have 2 Users, 1 is an Admin, and 1 is a Subscriber. You have to be logged in as an Admin in order for that Template selection to show up on the right side.