Hi Prashant,
We'll need some more information before we can help you with this:
What is the full error message you are seeing?
Who is logging the message (i.e. ACS or the device)?
What type of device are you trying to login to (i.e. ASA, router, etc.)?
What version of software does the device run?
What is the AAA configuration of the device?
Does ACS log any failed attempts when you try to login?
-Mike
