There are various scenarios when a Sitecore content author may be warned that an item will not publish. Read on to learn why:
Not in the final workflow step
If you publish now, the selected version will not be visible on the Web site because it is not in the final workflow step.
You will get the above message if the content item is is a workflow state that is not final. This is a friendly warning that you need to approve your content or have someone else that is an approver do so. This is the whole point of workflow, it prevents publishing until the final state.
Today does not fall within its valid date range
If you publish now, the selected version will not be visible on the Web site because today does not fall within its valid date range.
You will get the above message if the content item has publishing restrictions by date. To investigate the restrictions, use the View or Change button on the restrictions of the Publish tab.
It has been replaced by a newer version
If you publish now, the selected version will not be visible on the Web site because it has been replaced by a newer version.
You will get the above message if the content item has a newer version of the item that is approved for publish. Use the versions dropdown menu to select the newer version.
It has been replaced by an older version
If you publish now, the selected version will not be visible on the Web site because it has been replaced by an older version.
You will get the above message if the content item has a newer version that is not in the final state of workflow or that newer version is not marked as publishable. Thus, the older version will replace content when published because the newer version is not applicable for publish.
General Publishing Restrictions (item versions not selected)
If you publish now, the selected version will not be visible on the Web site.
You will get the above message if the content item has publishing restrictions that completely block it. This is likely the case where someone made the item always unpublishable for a reason. To investigate the restrictions, use the View or Change button on the restrictions of the Publish tab.
To learn more about workflow, visit the Sitecore Workflow documentation page.