Hi,
ok, what version of Dzen theme do you have? Have you tried to update theme?
If you want to update please make backup for each case.
When you go to “single column portfolio” you need to have only portfolio featured images, but when you go to “single column with youtube” video need to be there.
In the attachment you can find screenshot how that look from our side. Go to your Portfolio, add new and in right bottom corner box put Youtube Video ID or Vimeo Video ID.
If you don’t know how to do that, here is good explanation:
https://www.youtube.com/watch?v=EKyirtVHsK0
I hope this help you.
Kind regards