Maybe you could tell us what this ReportDocument
actually is rather than making us guess. I just searched on the Microsoft Docs site and it isn't listed there so it's obviously not a standard part of .NET, so it could come from anywhere. Provide a FULL and CLEAR explanation of the problem. What products and technologies you're using outside of the standard is always relevant.
That said, if you're referencing a library in a .NET Core project then it has to be built for .NET Core. You need to check for yourself first whether the library you're using was or not.