I tested the script from the website you gave, and it works fibe in varFDTD:
could you test line by line and check what happens? sometimes it will work if you reboot your computer.
The error message says no GDS file found, meaning no GDS file is created.
Please try this example.https://optics.ansys.com/hc/en-us/articles/360034927093-gdsopen
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