Hi @sudheeshk,
The file is in the autotest folder, but that autotest folder should be part of your system’s PATH variable (see point 3 of our SITL troubleshooting docs) if the setup was completed correctly (so it should still be able to find it). I believe the idea is it should be run from within the ArduSub folder, so that it finds the rest of the files correctly.
Which OS are you running on? If you’re on Ubuntu we’ve got an open pull request to update the install script just updated the install script so that it works properly on recent systems (20.04), so it might be worth pulling the latest updates from the Sub-4.0 branch. If you’re on Windows the install process is potentially also outdated - not sure about that one unfortunately.
As a side note, it’s useful if error messages/tracebacks can be put into code blocks, so they’re more clearly separated from the rest of the text. There are instructions for how to make them in the Formatting a Post/Comment section of the How to Use the Forums guide 
(I’ve done this one for you)