StopIteration, I don't know how to procede

I would like to give some feedback about ...

From: https://py.checkio.org/mission/power-plants/solve/

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Hello, my code at some of the extra tests can't found a combination of letters to solve the problems, I tried to find the cause of this but I ran out of ideas. please help.

my code: https://github.com/EdinsonUwU/POWER_PLANTS/tree/main

PS: I suspect it has to be something related to the way in which I am using the algo of search of neighboors