Hi abhishek
The user QPAC is saying that you're trying to allocate the task to a null user. I suspect that the LDAP query in the previous step is bringing back null data from your ldap query. It's very hard to know exactly what's going on without seeing the detail of what you're doing in your workflow.
My suggestion is to work on one bit at a time - first make sure that your LDAP query is bringing back a valid user name. One way to test this is to specify the user-name-variable as an "out" parameter, and invoke the workflow as a web service. Another way to do this is to download our variableLogger QPAC (it's a freebie), which will enable you to output the values of your process variables to a file or to the console.
I've also found that the Softerra LDAP browser is an invaluable tool for helping to get your LDAP queries correct.
Good luck!
Howard
http://www.avoka.com