I have a working release pipeline that I set up years ago.
However when I view the YAML I see
#Your build pipeline references an undefined variable named ‘Parameters.WebsiteName’. 
Create or edit the build pipeline for this YAML file, define the variable on the 
Variables tab. See https://go.microsoft.com/fwlink/?linkid=865972
steps:
- task: IISWebAppDeploymentOnMachineGroup@0
  displayName: 'IIS Web App Deploy'
  inputs:
    WebSiteName: '$(Parameters.WebsiteName)'
    VirtualApplication: betaapiforme
    Package: '$(System.DefaultWorkingDirectory)/_myapp/drop/ApiForMe.WebApi.zip'
    TakeAppOfflineFlag: True
    JSONFiles: |
     **/appsettings.json
I can see the website name in the UI for the pipeline.
It is not editable.

Why is the YAML showing a warning?