You want to delete an Email Template but that template was used to create existing emails in your instance.
It is not possible to delete an Email Template that is in use by emails in your Marketo instance, even if those emails are not approved. If you wish to delete the template, you will need to first delete the emails that use it. You can find these in the Used By tab in the template's information. Alternately, you can break the connection between the email and the template, which would allow you to keep the email but delete the template.