This error message is caused by not having a "+Values" or "+Lock" in cell A1 of at least one worksheet of a workbook containing Jet formulas (NL,NP,GL,NF). Please see Introducing Report Viewers for additional information on preparing reports for Viewers.
Complete text of error message is:
You are not licensed to Design reports. If you are a licensed Report Viewer, please tell your Report Designer that "Auto+Hide+Values" or "Auto+Hide+Lock" must be in cell A1 of at least one worksheet of this report the report must be saved in Report mode.