ASP.NET Core WebApp(.NET Core) missing DataAnnotations - asp.net

Question

Following situation:
I have a .sln with multiple projects (data-access, business-objects, business-logic, web-server).
The Web-Project is an ASP.NET Core Web Application using .Net-Core 1.1 with the individual user account authorization (Identity) and the "WebApplication"-Template.
The rest of the projects is using the .Net-Framework 4.6.2.
Now I am about to remove the DBContext from the web-server and instead use the same as in the data-access-project.
I set up my ApplicationDBContext accordingly and my User entity extends the IdentityUser. I also changed everything in the web-server-project to use the ApplicationDBContext (in my case called WTHListModell).
I also has the references to the projects WTHList.GO (business-objects) and WTHList.DZ (data-access).
Now here comes the problem:
Doing this using the full .NET-Framework for the Web-Server (creating a ".NET-Core Web-Application (.Net-Framework)" ) is successful and works just fine.
With .Net-Core 1.1 I am getting a FileNotFound-Exception regarding "System.ComponentModel.DataAnnotations" whenever I try to either log-in or register a user.
Now I have read, that DataAnnotations isn't compatible with .Net-Core and I also tried to add other NuGet-Packages and Assemblies to the Web-Project.
I don't know what else I can do about that, because I am supposed to use the .NET-Core Framework 1.1 for the Web-Projekt and it keeps on giving me this particular error.
Has anybody got a clue on how to solve this issue?
I have searched for a while now and I haven't found a solution for this yet...
And if it has any relevance: I am using the VS2017 Enterprise RC.
Thanks in advance.
UPDATE
This is the content of the csproj file:
<Project ToolsVersion="15.0" Sdk="Microsoft.NET.Sdk.Web">
<PropertyGroup Label="Globals">
<SccProjectName>SAK</SccProjectName>
<SccProvider>SAK</SccProvider>
<SccAuxPath>SAK</SccAuxPath>
<SccLocalPath>SAK</SccLocalPath>
</PropertyGroup>
<PropertyGroup>
<OutputType>Exe</OutputType>
<TargetFramework>netcoreapp1.1</TargetFramework>
<PreserveCompilationContext>true</PreserveCompilationContext>
</PropertyGroup>
<PropertyGroup>
<PackageTargetFallback>$(PackageTargetFallback);portable-net45+win8+wp8+wpa81;</PackageTargetFallback>
</PropertyGroup>
<PropertyGroup>
<UserSecretsId>aspnet-WTHList.Web2-ab5b6dbd-698d-4877-a2b3-6dd9b7a37960</UserSecretsId>
</PropertyGroup>
<ItemGroup>
<PackageReference Include="Microsoft.EntityFrameworkCore" Version="1.1.0" />
<PackageReference Include="Microsoft.NETCore.App" Version="1.0.1" />
<PackageReference Include="Microsoft.AspNetCore.Diagnostics" Version="1.0.0" />
<PackageReference Include="Microsoft.AspNetCore.Mvc" Version="1.0.1" />
<PackageReference Include="Microsoft.AspNetCore.Routing" Version="1.0.1" />
<PackageReference Include="Microsoft.AspNetCore.Server.IISIntegration" Version="1.0.0" />
<PackageReference Include="Microsoft.AspNetCore.Server.Kestrel" Version="1.0.1" />
<PackageReference Include="Microsoft.AspNetCore.StaticFiles" Version="1.0.0" />
<PackageReference Include="Microsoft.Extensions.Configuration.EnvironmentVariables" Version="1.0.0" />
<PackageReference Include="Microsoft.Extensions.Configuration.Json" Version="1.0.0" />
<PackageReference Include="Microsoft.Extensions.Logging" Version="1.1.0" />
<PackageReference Include="Microsoft.Extensions.Logging.Console" Version="1.0.0" />
<PackageReference Include="Microsoft.Extensions.Logging.Debug" Version="1.0.0" />
<PackageReference Include="Microsoft.Extensions.Options.ConfigurationExtensions" Version="1.0.0" />
<PackageReference Include="Microsoft.VisualStudio.Web.BrowserLink.Loader" Version="14.0.0" />
</ItemGroup>
<ItemGroup>
<PackageReference Include="Microsoft.AspNetCore.Authentication.Cookies" Version="1.0.0" />
<PackageReference Include="Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore" Version="1.0.0" />
<PackageReference Include="Microsoft.AspNetCore.Identity.EntityFrameworkCore" Version="1.0.0" />
<PackageReference Include="Microsoft.EntityFrameworkCore.Design" Version="1.1.0" />
<PackageReference Include="Microsoft.EntityFrameworkCore.SqlServer" Version="1.1.0" />
<PackageReference Include="Microsoft.EntityFrameworkCore.SqlServer.Design" Version="1.0.1" />
<PackageReference Include="Microsoft.EntityFrameworkCore.Tools" Version="1.1.0-preview4-final" />
<PackageReference Include="Microsoft.Extensions.Configuration.UserSecrets" Version="1.0.0" />
<PackageReference Include="Microsoft.VisualStudio.Web.CodeGeneration.Design" Version="1.0.0-msbuild2-final" />
</ItemGroup>
<ItemGroup>
<DotNetCliToolReference Include="Microsoft.EntityFrameworkCore.Tools.DotNet" Version="1.0.0-msbuild2-final" />
<DotNetCliToolReference Include="Microsoft.Extensions.SecretManager.Tools" Version="1.0.0-msbuild2-final" />
<DotNetCliToolReference Include="Microsoft.VisualStudio.Web.CodeGeneration.Tools" Version="1.0.0-msbuild2-final" />
</ItemGroup>
<ItemGroup>
<ProjectReference Include="..\WTHList.DZ\WTHList.DZ.csproj" />
<ProjectReference Include="..\WTHList.GL\WTHList.GL.csproj" />
<ProjectReference Include="..\WTHList.GO\WTHList.GO.csproj" />
</ItemGroup>
</Project>

Answer 1

Don't be confused with different "Core" terms: .NET Core is new "base" framework, which "looks like" .NET Framework 4.5 (4.6, etc), but this is other framework. You can compile multiple versions of same project (for different frameworks) at once.
ASP.NET Core is like old ASP.NET, but a new one. It is compiled twice - once for .NET Framework 4.5.1 (and "compatible" 4.5.2, 4.6, 4.6.1) and once for .NET Core (actually, for netstandart1.6 which is some kind of "specification", which is implemented by NET Core).
So, you can create web apps using ASP.NET Core which can build/run on .NET Framework 4.5.1 and/or on .NET Core. But you need all dependencies to be also build on (compatible with) .NET Framework 4.5.1 and/or on .NET Core.
So, if your app require some libraries (e.g. System.ComponentModel.DataAnnotations) which is not available for NET Core - you can't build/run app on .NET Core. You can only build ASP.NET Core app on top of .NET Framework 4.5.1+.