Emails not being sent when transaction complete
The error is not telling you that the email address is missing, I think, but rather that the email itself to send is missing.  

Have you assigned an actual email to send for that particular status?

Commerce->Settings->Order Statuses - check the column 'Has Emails' has a value of 1 or more in it.  

(To actually assign one, create it first in the email section, and then in this section click on the order status and in the bottom field 'Status Emails' you'll then be able to choose your created email).

Based on the error and the code generating it (in Commerce_OrderStatusesService.php) - I would say you've set up a status but not actually assigned the email you've made to it.