"Fertig" is more like "finished" therefore you can say:
"Ich bin fertig mit der Hausaufgabe und jetzt kann ich was anderes machen" = I am finished with the homework and I can do something else now.
"Ich bin fertig, ich kann nicht mehr laufen, ich muss eine Pause machen" = I am "done/finished" (with the meaning physically/psychically totally exhausted), I can't run anymore, I have to take a break.
Yeah, actually I was laughing so hard I had trouble writing in the translation.
Fertig means done. Ich bin fertig = I am done. Bereit means ready/prepared. Ich bin bereit = I am ready.
In this sense I guess they figured "I am done preparing" or something, which is really weird that they didn't just do "I am done". It's a much more correct translation (fertig doesn't mean ready), and it's used exactly the same way in German and English. If you weren't ready because you had to prepare, but now you are ready because you're done preparing, you'd say "I am done" to mean "I am ready". You wouldn't then say "done" means "ready".
Duolingo is a decent tool, but jeez if it isn't frustratingly bad sometimes. Perfectly acceptable translations are rated as unacceptable, while cases like this are presented without any explanation at all. On top of that you get things like "you're welcome" being exclusively "bitte" or "gern geschehen", despite "du bist willkommen" being correct in cases like "you're welcome (in this house)", and that too isn't explained.
"Fertig" seems to mean: "ready", "finished", and "exhausted". It seems confusing because these words aren't synonyms in English.
The way I understand it is, the words "finished" and "exhausted" do not refer to the upcoming action like the word "ready" does. Rather it refers to the previous action, such as saying:
"I am 'finished' with this current action therefore I'm 'ready' for this new action"
"My time on spent on this current action is 'exhausted' therefore I'm 'ready' for this new action"
Hope this helps(:
This is the second time that "fertig" sounded like "trautig". This Google Translate version sound much better to me.
See this Duo Tip: https://www.duolingo.com/skill/de/Not/tips-and-notes It tells us where "nicht" goes.
Except, were you referring to "not" or to "nicht"? IN English, "not" does not go at the end, usually.