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.
Click Yes.
More info about Assigning playlists to a channel is here.