When i upgrade ReportViewer from V11 to 15 Giving error

To easily upgrading from reportviewer version 11 to 15, you should consider to remove all old DLL of report viewer from your project and then you can install reportviewer version 15 using Nuget

Install-Package Microsoft.ReportingServices.ReportViewerControl.WebForms

Before installing above package also remove all reference of old Report viewer from Web.Config file.

Refer this article

https://docs.microsoft.com/en-us/sql/reporting-services/application-integration/integrating-reporting-services-using-reportviewer-controls-get-started?view=sql-server-ver15

To Have fresh installation of Reportviewer.