The base class includes the field 'ReportViewer1', but its type (Microsoft.Reporting.WebForms.ReportViewer) is not compatible with the type of control (Microsoft.Reporting.WebForms.ReportViewer)

This error normally occurs when different versions of Microsoft.ReportViewer.WebForms.dll are referenced in web.config and in controls.

Clean the solution and delete the related dll files and then again rebuild the solution