VSTS: Directory 'd:\a\1\a' is empty. Nothing will be added to build artifact 'drop'

You need to copy and add artifact, so for me I had to add this code to the end of my .yaml file

- task: CopyFiles@2
    targetFolder: '$(Build.ArtifactStagingDirectory)'    

- task: PublishBuildArtifacts@1    
  displayName: 'Publish Artifact: drop'
    PathtoPublish: '$(build.artifactstagingdirectory)'

First, remove ";" before Properties.EF6 (/t:;Properties.EF6;Sir.Domain).

Note: If Sir.WebUI dependency on Properties.EF6, Sir.Domain, Sir.EF6 and StandardClassLibrary, you just need to build Sir.WebUI project.

Secondly, for .Net Core task, change build command to publish and arguments is -o $(build.artifactstagingdirectory)\SIR.

Thirdly, remove Copy Files task (do not need to copy files to artifact).