'Restore failed for ... /Openchain/Openchain.csproj.' error when running OpenChain project

[eveyw]

Steps to reproduce

I was following the steps given by OpenChain platform (https://docs.openchain.org/en/latest/general/running-openchain.html). And coz the dotnet version specified in the original files is 1.0.4, so I did migration. After migration, 'dotnet restore' failed in .csproj file.

In Openchain.csproj:

<RuntimeFrameworkVersion>2.1.1</RuntimeFrameworkVersion>
<PackageTargetFallback>$(PackageTargetFallback);dotnet5.6;dnxcore50;portable-net45+win+wpa81+wp80+Xamarin.iOS10+MonoAndroid10+MonoTouch10</PackageTargetFallback>

<ItemGroup>
<PackageReference Include="Microsoft.AspNetCore.Server.IISIntegration" Version="1.0.2" />
<PackageReference Include="Openchain.Server" Version="0.6.2" />
<PackageReference Include="Openchain.Anchoring.Blockchain" Version="0.6.2" />
<PackageReference Include="Openchain.Sqlite" Version="0.6.2" />
<PackageReference Include="Openchain.SqlServer" Version="0.6.2" />
<PackageReference Include="Openchain.Validation.PermissionBased" Version="0.6.2" />

Expected behavior

The OpenChain APP restores successfully and runs.

Actual behavior

$ dotnet restore
Restoring packages for /Users/eveyw/Desktop/Blockchain/Openchain/Openchain.csproj...
/Users/eveyw/Desktop/Blockchain/Openchain/Openchain.csproj : error NU1102: Unable to find package Microsoft.NETCore.App with version (>= 2.1.1)
/Users/eveyw/Desktop/Blockchain/Openchain/Openchain.csproj : error NU1102: - Found 31 version(s) in nuget.org [ Nearest version: 2.1.0-preview2-26406-04 ]
/Users/eveyw/Desktop/Blockchain/Openchain/Openchain.csproj : error NU1102: - Found 2 version(s) in /usr/local/share/dotnet/sdk/NuGetFallbackFolder [ Nearest version: 2.0.6 ]
Generating MSBuild file /Users/eveyw/Desktop/Blockchain/Openchain/obj/Openchain.csproj.nuget.g.props.
Generating MSBuild file /Users/eveyw/Desktop/Blockchain/Openchain/obj/Openchain.csproj.nuget.g.targets.
Restore failed in 1.61 sec for /Users/eveyw/Desktop/Blockchain/Openchain/Openchain.csproj.

Environment data

dotnet --info output:

.NET Command Line Tools (2.1.105)

Product Information:
Version: 2.1.105
Commit SHA-1 hash: 141cc8d976

Runtime Environment:
OS Name: Mac OS X
OS Version: 10.13
OS Platform: Darwin
RID: osx.10.13-x64
Base Path: /usr/local/share/dotnet/sdk/2.1.105/

Microsoft .NET Core Shared Framework Host

Version : 2.0.7
Build : 2d61d0b043915bc948ebf98836fefe9ba942be11