Twilio was unable to process the correct response to your Function invocation because your version of Functions is incompatible with multi-valued HTTP response headers.
A Function invocation has resulted in a Twilio Response with multi-valued headers (i.e. the header value is an array). Multi-valued headers are not supported in Classic Functions, or in @twilio/runtime-handler dependency versions below 1.2.0.
Use the new Functions/Assets UI or Serverless API, and upgrade your @twilio/runtime-handler dependency version to 1.2.0 or higher. Please refer to this for more information about the Runtime Handler, how to upgrade the version, and the currently available versions. For Classic Functions, or incompatible Runtime Handler versions, only String-valued headers are permitted.