Hi Snehal,
This is a problem due to old ST-A/PI tool.
Before starting the upgrade, run transaction ST13 tool RTCCTOOL.It will show RED mark on ST-A/PI Addon Supportpackage.
Follow the SAP note described on that screen and update your ST-A/PI version as mentioned there using transaction SPAM.
After ST-A/PI upgraded to version required, you have to perform Maintenance optimizer again and create a new stack file.Start the the upgrade with new stack file
Best regards, Praveen