Drone CI/CD only stuck in exec pipeline

Question

When i use docker pipeline it will success building. But when i use exec pipe line it's always stuck in pending. And i don't what going wrong.

kind: pipeline
type: exec
name: deployment

platform:
  os: linux
  arch: amd64

steps:
- name: backend image build
  commands:
    - echo start build images...
    # - export MAJOR_VERSION=1.0.rtm.
    # - export BUILD_NUMBER=$DRONE_BUILD_NUMBER
    # - export WORKSPACE=`pwd`
    # - bash ./jenkins_build.sh
  when:
    branch:
      - master

Docker Pipe Line is fine.

kind: pipeline
type: docker
name: deployment

steps:     
- name: push image to repo
  image: plugins/docker
  settings:
    dockerfile: src/ZR.DataHunter.Api/Dockerfile
    tags: latest
    insecure: true
    registry: "xxx"
    repo: "xxx"
    username:
      from_secret: username
    password:
      from_secret: userpassword