The Delivery activity/step in a campaign workflow is blinking Red and the workflow journal provides no further error information.
An error has occurred during delivery preparation. To determine the cause, go to the Campaign dashboard and Open the delivery. A common error is missing subject line. The error shown here is that no content has been specified.
Correct the error that caused the delivery to fail.
Send a proof before executing the campaign. The proof is analyzed with the same set of rules as are applied to the Delivery (in particular the content rules, as the target for the Delivery is different) and you have a chance to correct errors before the campaign is due to go out.