How to access environment variables defined in Function Settings tab?

We cannot get the Functions environment variables to work as described in the “templating” section of the “project.yml” reference documentation (https://docs.digitalocean.com/products/functions/reference/project-configuration/#templating). When we set an environment variable under the DO platform’s Functions > [NAMESPACE] > [FUNCTION_NAME] > Settings, then try to deploy the function from doctl with templating in the project.yml file, it throws this error:

"Error: The following substitutions could not be resolved: 'SECRET_KEY'"

(‘SECRET_KEY’ being the name of the variable that we want to keep only on the DO platform and not in some config file for obvious reasons).

Here is the project.yml file for reference:

environment: {}
parameters: {}
packages:
  - name: sample
    environment: {}
    parameters: {}
    annotations: {}
    functions:
      - name: hello
        binary: false
        main: ''
        runtime: 'python:default'
        web: true
        parameters: {}
        environment:
          SECRET_KEY: "${SECRET_KEY}"
        annotations: {}
        limits: {}