Hi ladyfreak,

Can you please share your site credentials here, because we need to login to your site and see this issue live. Your information is safe (private) in this thread.

To include your credentials click Insert Credentials button in editor toolbar. (See how)

Hi again,

1. Menu is showing, the full-screen menu is showing and if you want to change the type of menu follow the documentation of the theme.

2. I have saved the Permalinks with the postname setting and it worked, the items are not showing 404 error anymore.

3. You can create a page and then edit the Portfolio and Design and link them to the pages you created, you can link them to an external page like this:

For each item you can set an action button and link the project with the link you provide here, also you will be able to add a custom external link to your item, this way visitors will be redirected  to the link you’ve entered here to do this simply make sure you’re editing on the Project Link tab.

• Launch Link Title — Set the title of the button/link you want to add.
• Link URL — Add the URL where you want to redirect users when the button is clicked.
• Link Target — Ticking it will open the link on a new tab/window.
• Item Linking — Select if you want to send users to the link you’ve entered above, this will not allow them to see the portfolio item but will redirect them immediately.

source: Documentation.