Cannot terminate EC2

If the EC2 instance has been in shutting down state, see this link

Specific section relevant to your issue:

Delayed instance termination

If your instance remains in the shutting-down state longer than a few minutes, it might be delayed due to shutdown scripts being run by the instance.

Another possible cause is a problem with the underlying host computer. If your instance remains in the shutting-down state for several hours, Amazon EC2 treats it as a stuck instance and forcibly terminates it.

If it appears that your instance is stuck terminating and it has been longer than several hours, post a request for help to the Amazon EC2 forum . To help expedite a resolution, include the instance ID and describe the steps that you've already taken. Alternatively, if you have a support plan, create a technical support case in the Support Center.

As for the "Server.InternalError: Internal error on launch" status, there could be few reasons, see this KB article