Azure CI pipeline for Blazor .NET 5 doesn't work

Change your UseDotNet task to the following in order to make sure that the Visual Studio 2019 preview is used in conjunction with .NET5 preview:

- task: UseDotNet@2
  displayName: 'Use .NET 5 SDK (preview)'
  inputs:
    packageType: 'sdk'
    version: '5.0.100-rc.1.20452.10'
    vsVersion: '16.8.0'
    includePreviewVersions: true

Full YAML pipeline for your reference (this is working for my Blazor WASM .Net 5 project):

pool:
  vmImage: 'ubuntu-latest'

steps:

- task: UseDotNet@2
  displayName: 'Use .NET 5 SDK (preview)'
  inputs:
    packageType: 'sdk'
    version: '5.0.100-rc.1.20452.10'
    vsVersion: '16.8.0'
    includePreviewVersions: true

- task: DotNetCoreCLI@2
  displayName: 'NuGet restore'
  inputs:
    command: 'restore'
    projects: 'MyProject/MyProject.csproj'
    verbosityRestore: 'Normal'

- task: DotNetCoreCLI@2
  displayName: 'Build'
  inputs:
    zipAfterPublish: true
    command: publish
    publishWebProjects: false
    projects: 'MyProject/MyProject.csproj'
    arguments: '-c $(Build.Configuration) -o $(Build.ArtifactStagingDirectory) --no-restore'

- task: PublishBuildArtifacts@1
  displayName: 'Publish'
  inputs:
    PathtoPublish: '$(Build.ArtifactStagingDirectory)'
    ArtifactName: 'drop'
    publishLocation: 'Container'