Q:

Build issue with nuget package

I am trying to deploy a React App on Visual Studio, but I am having issues with a nuget package. I keep getting the error:
Could not load file or assembly 'Microsoft.AspNetCore.Razor.Language, 
Version=2.1.1.0, Culture=neutral, PublicKeyToken=adb9793829ddae60'. 
The system cannot find the file specified.

How can I fix this?

A:

Build issue with nuget package

To resolve this issue, first, you should check if you can deploy to azure on local. If this issue also occurred on the local without visual studio online, you should check if this assembly is missing for your project, you can add this assembly by nuget.
If this issue not occurred on the local just for visual studio online, you may need to add a NuGet restore task to restore nuget package before you build and deploy your project:

See Restore Package Management NuGet packages in Team Build for more details.
If above not help you. Please share more build log when you deploying in the visual studio online (Hide personal information).

