How to check where are the bugs when the execution status is error

I want to execute a workflow,the workflow definition as follows:

---

version: '2.1.1'

send_tenant_stat:
 type: direct

 input:
  - to_email
  - from_email
  - smtp_server
  - smtp_password

 output:
  vm_count: $.vm_count
  active_vm_count: $.active_vm_count
  net_count: $.net_count

 tasks:
  get_vm_count:
   action: nova.servers_list
   publish:
    vm_count: $.length()
    active_vm_count: $[$.status = 'ACTIVE'].length()
   on-complete:
    - get_net_count
  get_net_count:
   action: neutron.list_networks
   publish:
    net_count: $.length()
   on-complete:
    - send_stat
  send_stat:
   action: std.email
   input:
    params:
     to: [$.to_email]
     suject: Tenant Statistics
     body: |
      Statistics for tenant '{$.openstack.project_name}'
      Number of VMs: {$.vm_count}
      Number of active VMs: {$.active_vm_count}
      Number of networks: {$.net_count}

    settings:
     from: $.from_email
     smtp_server: $.smtp_server
     password: $.smtp_password

and,my input is:
 {"to_email":"<email address hidden>","from_email":"<email address hidden>","smtp_server":"smtp.gmail.com","smtp_password":"my email's password"},and store it in Tenant_Statistics.json

then,I run the following command to execute the workflow:
mistral execute-create XXX(workflow's name) Tenant_Statistics.json

but the status is error ,and the output is None.