The runtime used in the Serverless Build request is not supported on the platform. A runtime defines the environment your Functions will be executed in (e.g. which Node.js version to use for your Functions).
Possible Causes
A Serverless Build request was made without providing the runtime and the runtime used for the last successful build in the service (e.g. a Node.js version that is no longer supported on the platform).
A Serverless Build request was made with a specified runtime (e.g. node10) that is currently not supported on the platform.