这个问题可能由于缺少AWS CDK的依赖项导致，可以在pipeline中添加一个安装依赖项的步骤。可以在pipeline中执行以下命令：

npm install -g aws-cdk
npm install

如果出现权限问题，可以在Pipeline YAML文件中的环境变量中添加AWS密钥并进行授权。

示例代码：

trigger:
  branches:
    include:
    - master

pool:
  vmImage: 'ubuntu-latest'

variables:
  # AWS credentials
  AWS_ACCESS_KEY_ID: $(AWS_ACCESS_KEY_ID)
  AWS_SECRET_ACCESS_KEY: $(AWS_SECRET_ACCESS_KEY)
  AWS_REGION: $(AWS_REGION)

steps:
- task: NodeTool@0
  inputs:
    versionSpec: '10.x'
  displayName: 'Install Node.js'

- script: |
    npm install -g aws-cdk
    npm install
  displayName: 'Install CDK and Dependencies'

- script: |
    cdk synth
  displayName: 'Synthesize AWS CloudFormation Stack'