Hi,
Try to remove the reference dll and any assemblies entry of ReportViewer from web.config. Add new reference of ReportViewer and rebuild the project.
Or try to compare between <%@ Register Assembly="Microsoft.ReportViewer.WebForms, Version=8.0.0.0 and the version that is written in the reference you added in the web.config file, if there is a difference in version of the Microsoft.ReportViewer.WebForms then change as per the version installed in the server.