Looks like perhaps the context that is executing the RollUp service doesn't have visibility rights to this other service it is calling.
Thanks for the inputs .It helped me resolving the error . Yes I had to correct the visibility rights and also a small correction in the script as I was not using the demo version of Thingworx.
var sysServThingName = "PowerSystemServices" + "_" + username;
corrected the above statement to
var sysServThingName = "PowerSystemServices";
@anitha_s_kumar Thanks for letting us know. Please indicate the appropriate response as the Accepted Solution for the benefit of other Community members who have the same question.