Unable to initialize my CloudHSM cluster even though AWS says it’s available US-EAST-1. Get error on my Private SN. “CloudHsmInvalidRequestException – CloudHSM is not yet available in the us-east-1c zone.”

Try a different AZ.  "AWS CloudHSM might not be available in all Availability Zones in a given Region. However, this should not affect performance, as AWS CloudHSM automatically load balances across all HSMs in a cluster."  https://docs.aws.amazon.com/cloudhsm/latest/userguide/regions.html