I looked a little down the technical rabbit-hole to find least is an adverb while less is a determiner.
Brief reading seems to say that grammatically adverbs modify verbs... while determiners modify nouns... (while not being adjectives). And successful is an adjective. So... uh... wait?
Maybe I should go for examples.
If you said "Looking back at the lives of Alice, Bob, and Eve - considering how close they came to accomplishing their goals - it was Bob who was the least successful." I'd have no problem understanding that. But note that "least" is a superlative...it is saying that someone is at an extreme; they are the "best" or they are the "worst". For a sentence to make sense you must know about the context. (We've listed three people and you can see the two people that Bob came in last when he was ranked against them.)
It's actually valid to say "Bob is the least successful person I know." That's because it is clear from context the set of people you are comparing Bob against: everybody you know. So the superlative is resolved.
But you could not say "Bob is the less successful person I know." You'd have to say something like "Between Alice and Bob, I would say Bob is the less successful person."
So this brings us back to your sentence, with spelling errors corrected:
Hunting and fishing techniques were highly developed among the North American Indians, particularly in regions where agriculture was least successful.
One issue that is a little "nails on a chalkboard" here is that you are defining multiple things ("regions") as being superlatively the "least successful". Imagine this sentence:
Hunting and fishing techniques were highly developed among the North American Indians, particularly in the region where agriculture was the least successful.
That "the" thrown in is optional. In any case, dealing with a more obviously singular case makes "least" feel more natural. Although here it's hard to know what the regions we are comparing with would be, context suggests "the regions of North America we are discussing". So it is comfortable.
Now how about that substitution:
Hunting and fishing techniques were highly developed among the North American Indians, particularly in regions where agriculture was less successful.
What makes this better to me is the plurality of "regions". "The north regions were less successful than the south regions." makes sense. But "The north regions were least successful than the south regions" does not.
Anyway it is only this plurality issue that makes the sentence you give sound awkward, in my view. The only thing you need to do is take care about that distinction of what a superlative is; you really are putting something at the bottom of the heap when you use "least"... while a word like "less" is not making such a strong statement.
I can't entirely map out the adverb/determiner implications in any kind of clear way other than say "go read those articles"...but of course as a native speaker I did not learn the language by a schematic. The rules kind of emerge in your head somehow. It's starting to bug my programmer brain. :-/