From the link that BernieHunt already gave you, three possible reason:
ReportViewer could not find the .rdlc corresponding to the subreport. One or parameters expected by the subreport
were not passed by the main
Data for the subreport was not supplied. (Did you forget to handle the SubreportProcessing event?)
Yours is likely reason #3. If you don't supply the data for the subreport in
SubreportProcessing event (sample code provided), then you'll get this error.