See the document for the relational database management system that hosts the data warehouse.
Validate the reports as and when they are created by using the Validate Report feature in the Report Studio tools menu.
When creating expressions, before you close the editor, validate the expression by using the Validate feature.
See the Troubleshooting section in IBM® Cognos® Report Studio User Guide, IBM Cognos Administration and Security Guide, and other IBM Cognos documentation.