Build error - The type or namespace name 'AspNetCore' does not exist in the namespace 'Microsoft' (are you missing an assembly reference?

I have just opened a project that was working fine the other day. No changes were made to the code but now when I open it I get errors

The type or namespace name 'AspNetCore' does not exist in the namespace 'Microsoft' (are you missing an assembly reference?

The only thing that has changed is VS has upgraded to 16.5

How do I fix this? The packages are all there and Nuget reports everything is installed. I have tried uninstalling the packages and reinstalling them but still get the error.

This is my .csproj file

<Project Sdk="Microsoft.NET.Sdk.Web">

  <PropertyGroup>
    <TargetFramework>netcoreapp3.1</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.AspNetCore.Authentication.JwtBearer" Version="3.1.2" />
    <PackageReference Include="Microsoft.AspNetCore.Mvc.Cors" Version="2.2.0" />
    <PackageReference Include="Microsoft.EntityFrameworkCore" Version="3.1.2" />
    <PackageReference Include="Microsoft.VisualStudio.Web.CodeGeneration.Design" Version="3.1.1" />
  </ItemGroup>


</Project>