Navigaton Menu Will Not Connect to Category Pages

I am a relative beginner at using wordpress.org software, so please bear with me if I forget to include all of the information I should. We recently installed the Sparc theme, though we are using the purchased version. Everything is functioning as we need it to, other than the secondary navigation menu. While the categories are displayed, when you click on them, it displays “Internal Server Error.” I have tried following these troubleshooting guides,

http://www.wpbeginner.com/wp-themes/how-to-add-custom-navigation-menus-in-wordpress-3-0-themes/

https://codex.wordpress.org/Navigation_Menus”

but plugging in the code either did not work, or I pasted it in the wrong place.

Can anyone tell me why the category menu does not navigate to a page with posts that fall within that category?

Here is the link to our site: http://www.alaskacommons.com/

Any suggestions are welcome. Thank you for your help.