yes this worked, you need to add the variables at both places i.e. the top level and set default values, and then add them again to the target as a mapping ( non default values).

bundle: # Required.
  name: string # Required.
  databricks_cli_version: string
  cluster_id: string
  git:
    origin_url: string
    branch: string

# These are for any custom variables for use throughout the bundle.
variables:
  <some-unique-variable-name>:
    description: string
    default: string or complex

targets:
  <some-unique-programmatic-identifier-for-this-target>:
    artifacts:
      # See the preceding "artifacts" syntax.
    bundle:
      # See the preceding "bundle" syntax.
    cluster_id: string
    default: true | false
    mode: development
    presets:
      <preset>: <value>
    resources:
      # See the preceding "resources" syntax.
    sync:
      # See the preceding "sync" syntax.
    variables:
      <preceding-unique-variable-name>: <non-default-value>
    workspace:
      # See the preceding "workspace" syntax.
    run_as:
      # See the preceding "run_as" syntax.