As of current version, you can use DocumentUltimate only in a "ASP.NET Core on .NET Framework" project for now. Are you sure you have the correct project type?
Note that "ASP.NET Core on .NET Core" is a different platform and it's not supported yet because it requires porting all the code from .NET Framework runtime to .NET Core runtime so it requires a new DLL and .NET Core runtime does not provide all of the APIs yet. Minimum supported version is ASP.NET Core MVC 2.0.3 on .NET Framework 4.6.1 (this is because 2.0.3 fixes a bug related to referencing external DLLs in a razor page).
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