Code generation fails on heroku env

[supermomonga]

-----> ASP.NET Core app detected
Installing dotnet
publish /tmp/build_a47cf451cea5e8578982a2211441cec6/MoEmbed.App/MoEmbed.App.csproj for Release
Welcome to .NET Core 3.0!
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SDK Version: 3.0.100
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Explore documentation: https://aka.ms/dotnet-docs
Report issues and find source on GitHub: https://github.com/dotnet/core
Find out what's new: https://aka.ms/dotnet-whats-new
Learn about the installed HTTPS developer cert: https://aka.ms/aspnet-core-https
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Microsoft (R) Build Engine version 16.3.0+0f4c62fea for .NET Core
Copyright (C) Microsoft Corporation. All rights reserved.
  Restore completed in 139.35 ms for /tmp/build_a47cf451cea5e8578982a2211441cec6/MoEmbed.CodeGeneration/MoEmbed.CodeGeneration.csproj.
  Restore completed in 600.91 ms for /tmp/build_a47cf451cea5e8578982a2211441cec6/MoEmbed.Core/MoEmbed.Core.csproj.
  Restore completed in 13.7 ms for /tmp/build_a47cf451cea5e8578982a2211441cec6/MoEmbed.Models/MoEmbed.Models.csproj.
  Restore completed in 437.14 ms for /tmp/build_a47cf451cea5e8578982a2211441cec6/MoEmbed.Twitter/MoEmbed.Twitter.csproj.
  Restore completed in 1.43 sec for /tmp/build_a47cf451cea5e8578982a2211441cec6/MoEmbed.App/MoEmbed.App.csproj.
  Restore completed in 2.05 sec for /tmp/build_a47cf451cea5e8578982a2211441cec6/MoEmbed.CodeGeneration/MoEmbed.CodeGeneration.csproj.
  Generate OEmbed proxy metadata providers.
  It was not possible to find any compatible framework version
  The specified framework 'Microsoft.NETCore.App', version '2.1.0' was not found.
    - The following frameworks were found:
        3.0.0 at [/app/tmp/cache/dotnet/3.0.100/sdk/shared/Microsoft.NETCore.App]
  
  You can resolve the problem by installing the specified framework and/or SDK.
  
  The .NET Core frameworks can be found at:
    - https://aka.ms/dotnet-download
/tmp/build_a47cf451cea5e8578982a2211441cec6/MoEmbed.CodeGeneration/MoEmbed.CodeGeneration.csproj(23,5): error MSB3073: The command "dotnet script OEmbedProxyMetadataProviders.csx" exited with code 150.
  MoEmbed.Models -> /tmp/build_a47cf451cea5e8578982a2211441cec6/MoEmbed.Models/bin/Release/netcoreapp3.0/MoEmbed.Models.dll
  Successfully created package '/tmp/build_a47cf451cea5e8578982a2211441cec6/MoEmbed.Models/bin/Release/MoEmbed.Models.1.0.0.nupkg'.
 !     Push rejected, failed to compile ASP.NET Core app.
 !     Push failed

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The specified framework 'Microsoft.NETCore.App', version '2.1.0' was not found.

なぜ 2.1.0 が