This is exactly the problem. It’s a Python error, not a problem with the terminal at all.
The problem is actually in the way the OP is using the script.
What is arpDetector.py? Where did you get it from? It seems pretty obvious that you didn’t write that script. Because if you had, you would understand enough about Python to know why the script is failing to run.
Python is complaining that whatever is in that script isn’t a directly runnable module because it doesn’t have a __main__ module defined in it.
So it probably contains a class, or module that you can import and use in other Python scripts.
So you’ll probably need to create a script of your own and import whatever module/s are in that script. Then you’d need to create an instance of whatever objects you need to use.
But without seeing the actual contents of the script, it’s difficult to advise you on exactly what to do. Unfortunately my crystal ball is currently in for its yearly service, ha ha!
If you can let us know where you got the script from, so we can take a look at it. Or if you can’t remember where you got it from, then copy/paste the code in code tags in a new post, then we can see what’s going on in the script and someone can advise you on its correct usage.
It almost certainly needs to be imported into another script, but without seeing what classes/modules are available in the script, it’s impossible to give accurate advice!