DOCX rendering format is not available.

This error happens when a Telerik report is to be exported to Word document programmatically.

Take a look if the following DLLs are referenced:
Telerik.Reporting.OpenXmlRendering.dll
DocumentFormat.OpenXml.dll
Telerik.Reporting.XpsRendering.dll (for Xps file)

Note:
If just adding the DocumentFormat.OpenXml.dll does not work, then, the
Open XML SDK 2.0 for Microsoft Office must be installed first on the server.

Reference:
http://www.telerik.com/community/forums/reporting/telerik-reporting/report-export-to-docx.aspx