This is a very common error and you can find lots of threads about it by searching for yolo_non_max_suppression. But did you read all the way through the thread you linked above? E.g. here’s a later post on that thread that covers more issues. And even later on that thread, here’s a very comprehensive post from mentor Raymond. Definitely worth a look, but you will need to have your thinking cap firmly affixed and your morning coffee already on board! 
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