This error says that Google’s SDTT requires the url property for an Organization item. The value would be the URL of the organization (typically the homepage), not the URL of the organization’s logo.
Note that "requires" does not mean that you must provide the url property to have valid Microdata + Schema.org (neither Schema.org nor Microdata require any properties). It’s only Google that requires it for showing something based on your markup in Google Search (e.g., a Rich Snippet, or a Knowledge Graph feature like Corporate Contacts). If you don’t provide it, Google probably doesn’t show it, but nothing else happens.
