dotnet build - get error "could not return the default page '/index.html'"

The problem come from the fact when you build the project with the command dotnet build, the wwwroot folder is not copied.

I reproduced your problem, to fix it, I copied the folder in the output and it worked.

You should use the publish command to test production.

In either cases, both commands don't copy the wwwroot folder.

check angular.json file use "outputPath": "dist",

And Startup file

 services.AddSpaStaticFiles(configuration =>
      {
        configuration.RootPath = "ClientApp/dist";
      });

Resolved this issue by comparing existing Angular Asp net core template, I did following code changes.

In csproj file:

<TypeScriptCompileBlocked>true</TypeScriptCompileBlocked>
 <TypeScriptToolsVersion>Latest</TypeScriptToolsVersion>
 <IsPackable>false</IsPackable>
 <SpaRoot>ClientApp\</SpaRoot>
 <DefaultItemExcludes>$(DefaultItemExcludes);$(SpaRoot)node_modules\**</DefaultItemExcludes>

 <!-- Set this to true if you enable server-side prerendering -->
 <BuildServerSideRenderer>false</BuildServerSideRenderer>
 <UserSecretsId>baa2f579-d2a7-4eb4-89fb-1c171b2e2482</UserSecretsId>
 <TypeScriptToolsVersion>3.0</TypeScriptToolsVersion>

<ItemGroup>
 <TypeScriptCompile Include="App\src\app\models\user.ts" />
 <TypeScriptCompile Include="App\src\app\utils\AppContants.ts" />
 <!-- Don't publish the SPA source files, but do show them in the project files list -->
 <Content Remove="$(SpaRoot)**" />
 <None Include="$(SpaRoot)**" Exclude="$(SpaRoot)node_modules\**" />
</ItemGroup>

<ItemGroup>
 <Folder Include="Properties\PublishProfiles\" />
</ItemGroup>

<Target Name="DebugEnsureNodeEnv" BeforeTargets="Build" Condition=" '$(Configuration)' == 'Debug' And !Exists('$(SpaRoot)node_modules') ">
 <!-- Ensure Node.js is installed -->
 <Exec Command="node --version" ContinueOnError="true">
   <Output TaskParameter="ExitCode" PropertyName="ErrorCode" />
 </Exec>
 <Error Condition="'$(ErrorCode)' != '0'" Text="Node.js is required to build and run this project. To continue, please install Node.js from https://nodejs.org/, and then restart your command prompt or IDE." />
 <Message Importance="high" Text="Restoring dependencies using 'npm'. This may take several minutes..." />
 <Exec WorkingDirectory="$(SpaRoot)" Command="npm install" />
</Target>

<Target Name="PublishRunWebpack" AfterTargets="ComputeFilesToPublish">
 <!-- As part of publishing, ensure the JS resources are freshly built in production mode -->
 <Exec WorkingDirectory="$(SpaRoot)" Command="npm install" />
 <Exec WorkingDirectory="$(SpaRoot)" Command="ng build --prod" />
 <Exec WorkingDirectory="$(SpaRoot)" Command="npm run build:ssr -- --prod" Condition=" '$(BuildServerSideRenderer)' == 'true' " />

 <!-- Include the newly-built files in the publish output -->
 <ItemGroup>
   <DistFiles Include="$(SpaRoot)dist\**; $(SpaRoot)dist-server\**" />
   <DistFiles Include="$(SpaRoot)node_modules\**" Condition="'$(BuildServerSideRenderer)' == 'true'" />
   <ResolvedFileToPublish Include="@(DistFiles->'%(FullPath)')" Exclude="@(ResolvedFileToPublish)">
     <RelativePath>%(DistFiles.Identity)</RelativePath>
     <CopyToPublishDirectory>PreserveNewest</CopyToPublishDirectory>
   </ResolvedFileToPublish>
 </ItemGroup>
</Target>

In Startup.cs

services.AddSpaStaticFiles(configuration =>
            {
                configuration.RootPath = "ClientApp/dist";
            });

 app.UseSpa(spa =>
            {
                spa.Options.SourcePath = "ClientApp";
                if (env.IsDevelopment())
                {
                    spa.UseAngularCliServer(npmScript: "start");
                }
            });

In Angular angular.json

 "options": {
    "outputPath": "dist",