is there adequate free disk space on the Target?
does the SYSTEM account on the target have full permissions to %systemroot% (e.g., C:\Windws)
If the above items don't appear to be the problem:
Stop/Start the UEAgent service on the Master Agent machine, or
Reboot the Master Agent machine
As an unrelated issue, what will Prism do if the computer is off and an install is scheduled?