Namespace prefix ‘ref’ used but not declared in XML Publisher reports

Solution: This error will occur bevause of the higher version of the BI Publisher.

steps to resolve the error.

Step1: Open the rtf file, click on BI Publisher tab -> Options -> click on Build tab
-> set Form field size as "Backward Compatible" 

Step2: remove all the fields on rtf template and save then insert the fields and upload the rtf template. you will not
get the error called "Namespace prefix ‘ref’ used but not declared in XML Publisher"