Manasi,
For this one, i think you actually need to enter the API key in the Mashup itself (not the configuration of the mashup you have pictured)
Go to "Application and User Management" -> "Mashup Authoring"
You should see one titled "Embedded Map (bing.com)". When you select it, you'll see a button that says "Enter API Keys". Click that, and enter the appropriate API Key.
If you don't have one, you'll need to sign up for a Microsoft developer account to provide you with access (see the URL in the error message)
