Hi there @Venugopal ,
According to the precedence order of databricks overrides value should be taken first than default values

Anyways
If you just put the variable names in databrics.yml file it will work. No need to give default values if not required

 variables:
    v_task_key:
      description: The task key.
    v_job_cluster_key:
      description: The cluster key.
    v_job_name:
      description: The job name.