A. AWS does not currently have enough capacity to service the request for that instance type. A different Availability Zone or instance type must be used.
B. The account has reached its concurrent running instance limit. An EC2 limit increase request must be filed with AWS Support.
C. The APIs that service the EC2 requests have received too many requests and capacity has been reached. The request should be attempted again in a few minutes.
D. The Administrator did not specify the correct size of the instance to support the capacity requirements of the workload. Select a bigger instance.