I'm working on a pre-existing python code-by-zapier zap. The trigger is "Code By Zapier; Run Python". I've made some changes to the contained python script, and now when I go to test that step I run into the following error message:

We couldn’t find a run python
Create a new run python in your Code by Zapier account and test your trigger again.

Is there any way of figuring out what went wrong?
I'm guessing a little bit, but I think this issue stems from repeatedly testing an existing trigger without returning a new ID.
When you run a test (or click the "load more" button), then Zapier runs the trigger and looks through the array for any new items it hasn't seen before. It bases "newness" on whether it recognizes the id field in each returned object.
So if you're testing code that changed, but is returning objects with previously seen ids, then the editor will error saying that it can't find any new objects (the can't find new run pythons is a quirk of the way that text is generated; think of it as "can't find objects that we haven't seen before).
The best way to fix this depends on if you're returning an id and if you need it for something.

Your code can return a random id. This means every returned item will trigger a Zap every time, which may or may not be intended behavior.
You can probably copy your code, change the trigger app (to basically anything else), run a successful test (which will overwrite your old test data), and then change it back to Code by Zapier and paste your code. Then you should get a "fresh" test. Due to changes in the way sample data is stored, I'm not positive this works now
Duplicate the zap from the "My Zaps" page. The new one won't have any existing sample data, so you should be able to test normally.
How to troubleshoot "We couldn’t find a run python"?