Why i am unable to sign in using arcgis online account
Hello Satyam,
You need to have ArcGIS organizational account in order to work with Operation Dashboard for ArcGIS. You cannot login using the public account.
Create account—ArcGIS Online Help | ArcGIS
For a start, You can create a 60 days trial organizational account.
http://www.arcgis.com/features/free-trial.html