Hello,
Thank you for reaching out to us.
1/ To resolve the 404 error, please navigate to Settings > Permalinks in your WordPress dashboard and simply re-save the permalinks. After that, you can go to Brands and view the brand page to ensure everything is working correctly.
2/ Go to Header Builder > edit template > add Mega Menu element and then select template for menu item – https://prnt.sc/aZq3gk9MA6_l
Nav Menu element is used in your header content https://prnt.sc/Yw89cPcCErdI , in this case, this video will be helpful – https://youtu.be/JiI6qDaVtY4?feature=shared
Warm Regards,
The 8Theme Team