Reference Material:

https://docs.aws.amazon.com/AWSEC2/latest/WindowsGuide/troubleshooting-launch.html#troubleshooting-launch-capacity
Solution
To resolve the issue, try the following:
Wait a few minutes and then submit your request again; capacity can shift frequently.
Submit a new request with a reduced number of instances. For example, if you're making a single request to launch 15 instances, try making 3 requests for 5 instances, or 15 requests for 1 instance instead.
****If you're launching an instance, submit a new request without specifying an Availability Zone.
****If you're launching an instance, submit a new request using a different instance type (which you can resize at a later stage). For more information, see Change the instance type.
If you are launching instances into a cluster placement group, you can get an insufficient capacity error. For more information, see Placement group rules and limitations.