AWS announces preview of AWS Interconnect - multicloud
AWS announces AWS Interconnect – multicloud (preview), providing simple, resilient, high-speed private connections to other cloud service providers. AWS Interconnect - multicloud is easy to configure and provides high-speed, resilient connectivity with dedicated bandwidth, enabling customers to interconnect AWS networking services such as AWS Transit Gateway, AWS Cloud WAN, and Amazon VPC to other cloud service providers with ease.
Is there an additional section where you define the Stage parameter? It also appears that you are using JSON syntax for the Resource value while the overall document is YAML. Check the examples here: https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/mappings-section-structure.html#mappings-section-structure-examples