In order to help, we would like to see the project or at least the error that is thrown by the application. Please open a support ticket and send us a runnable sample of your project so we can examine it on our side.
The ReportDesign controller requires Telerik.WebReportDesigner.Services and Telerik.Reporting.Services.AspNetCore assemblies. They can be obtained via Nuget or referenced from the \bin\netstandard2.0 folder in Telerik Reporting installation directory.
The easiest way to see the Web Report Designer in action is to open our CSharp.AspNetCoreDemo from \Examples folder - usually in C:\Program Files (x86)\Progress\Telerik Reporting R3 2019\Examples\CSharp\Asp.NetCoreDemo. Under wwwroot node there is a file named webReportDesigner.html. Browse it and you should see a page with the loaded Web Report Designer.
Hope this helps.
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