First, you need to check the target framework of your project by following steps:
Right-click Project -> Properties ->Application.
If it is a Windows Forms App(.Net Core), there is no designer by default.
The .NET Core Windows Forms designer is available with the Visual Studio 16.5 Preview 1.
So you can try to update Visual Studio to Visual Studio 16.5 Preview 1 or a later version.
More details you can refer to this blog.
If it is .Net framework, you can open designer view via the steps below:
1.Open the solution explorer (View -> Solution Explorer, or Ctrl+W, S).
2.Select one of the mentioned classes like form or user control.
3.Right-click it and select View Designer (or simply Shift+F7). That will move you to the designer view.
For first error, the cause of the error is mostly missing mscorlib.dll.
And you can try the following suggestions to solve it.
1.Delete bin and obj file and rebuild your project.
2.Run dotnet restore command via Visual Studio -> Tools -> NuGet Package Manager -> Package Manager Console, then rebuild solution.
For second error, are you looking for the Task in System.Threading.Tasks?
Full reference is System.Threading.Tasks.Task, or for a using System.Threading.Tasks in your project