Make sure your content does not belong to a channel. You should change your content with another from all channels before you delete.
- Go to Channels.
- Type the name of your content you want to delete.
- You will see the list of channels that are using your content at the moment.
- Click on one of these channels.
- Change the content of this channel with another content and save changes.
- Repeat last step for every channel.
- Go to the content you want to delete.
- Click Delete icon.
- Click Yes.
More info about Assigning playlists to a channel is here.