To use Bing Map as a background layer, you must enter a valid Bing Map Key.

The Bing Map Key is stored as a global setting for this product.

You cannot enter in a blank string. If you enter a blank string, a red error icon will appear to the right of the text box with the tooltip "The value cannot be blank.".

To create a Bing Map key, see the following page: https://msdn.microsoft.com/en-us/library/ff428642.aspx.