Cannot modify an evaluated object originating in an imported file

Restarting Visual Studio fixed the issue for me.

I ran across this question as well as a couple others related to the error message, specifically. My issue was different from the OP's as it had to do with NuGet packages not installing/uninstalling/updating.

I opened the .CSProj files only to see the package reference completely missing for Microsoft.NET.Test.Sdk, ie some VS file somewhere was caching this bad reference, even after killing all existing .vs directories. So I just added it manually, ran nuget restore, and was back in business.

Basically, I modified this block of project file XML:

  <ItemGroup>
    <PackageReference Include="Microsoft.NETCore.UniversalWindowsPlatform">
      <Version>6.2.8</Version>
    </PackageReference>
    <PackageReference Include="MSTest.TestAdapter">
      <Version>1.4.0</Version>
    </PackageReference>
    <PackageReference Include="MSTest.TestFramework">
      <Version>1.4.0</Version>
    </PackageReference>
  </ItemGroup>

to this:

  <ItemGroup>
    <PackageReference Include="Microsoft.NETCore.UniversalWindowsPlatform">
      <Version>6.2.8</Version>
    </PackageReference>
    <PackageReference Include="MSTest.TestAdapter">
      <Version>1.4.0</Version>
    </PackageReference>
    <PackageReference Include="MSTest.TestFramework">
      <Version>1.4.0</Version>
    </PackageReference>
    <PackageReference Include="Microsoft.NET.Test.Sdk">
      <Version>16.1.1</Version>
    </PackageReference>
  </ItemGroup>