Is it possible to view the available capacity before submitting a capacity reservation request?

Hi,

The short answer is NO, this is not an external metric, if you do happen to run into ICE (Insufficient Capacity Errors) you can:

Either try again at a later time, try a different Availability Zone, or try a smaller capacity. If your application is flexible across instance types and sizes, try different instance attributes. as mentioned here.

Depending on your support plan your TAM can also assist with CR