Can you confirm whether this HTTPs endpoint for which you are trying to create subscription for you SNS Topic is private endpoint or is accessible over outside internet.
Amazon SNS functioning is designed in such a way that if you're subscribing an HTTP endpoint that isn't publicly accessible to an SNS topic, then you receive the following error :
"An error occurred (InvalidParameter) when calling the Subscribe operation: Invalid parameter: Unreachable Endpoint"
I would like to share that, currently SNS needs HTTP/S endpoints to be available on a public network before it can successfully subscribe them to the SNS Topic. Private HTTP/S endpoints are not directly supported in the SNS currently.
One more thing to confirm, the security-group for your EDM SOFTWARE, does it allow inbound traffic from all IPv4 address for port 443.
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