If you close the configuration dialog and hover the mouse over the "⚠️" icon on that processor, you will see a popup which enumerates the issues to be resolved. For the InvokeHTTP processor, it's likely that you have not specified a destination for all relationships -- you probably want a destination for Response (the response from the external web service), and possibly Retry and Failure. For any relationships you do not care about (Original, No Retry, etc.), you can "auto-terminate" those relationships from the "Settings" tab of the processor configuration dialog.
You can always see the required relationships from the Settings tab or by looking at the processor documentation (available online or embedded in the application by right clicking the processor and selecting "View Usage" from the context menu).
