"That length does not suffice."
Translation:Die Länge reicht nicht.
I probably would have rejected it too. But I can't explain why, except that I know it. ;-)
"genug" translates to "sufficient, enough" thats fine, I tried to find a rule but couldnt really find one as yet.
But the use of "genug" in following examples is correct.
If a measurement/dimension (the length) is sufficient you may use:
"Die Laenge ist ausreichend", "... es ist lang genug", "die Laenge genuegt den Anforderungen" "Die Laenge reicht aus.
DUO dropped the aus in the translation, which is not correct in my eyes.
Put it that way "gross" as an adjective for "Laenge" is not the best word choice as it is sort of a double-up.
On the other hand:
these examples are possible.
I hope that helped.
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Other explanations are also available on the web. For example: http://www.nthuleen.com/teach/grammar/negationexpl.html
In this sentence, it's at the end because it's negating the verb.