If I'm not mistaken, ‘er will nicht weiteres Brot’ could only be used when followed by ‘sondern’, because it is specifically negating ‘(additional) bread’, e.g.: ‘er will nicht weiteres Brot, sondern Wasser’ (‘he doesn't want any more bread, but rather water’). You could say ‘er will weiteres Brot nicht’, I'm not sure what the difference would be with the sentence with ‘kein’—if any—, but that is the only position where ‘nicht’ makes sense in the sentence because it should be negating it in its entirety.
earlier when i translated "Er will weiteres Brot." as "he wants another bread" duo made me use "he wants another loaf" but now for "Er will kein weiteres Brot." when I typed "he doesn't want another loaf" duo is making me go with "he doesn't want another bread". it just seems like it's tricking me in order to make me make a mistake, I don't think that remembering which questions accept which translations is constructive way of learning foreign language. both should be accepted in both cases.
“Bread” is an uncountable noun in English, you can't have “one bread”, you can have “bread” in general or “one loaf of bread”. “Brot” on the other hand can be used both in general as an uncountable noun and as a countable noun meaning “loaf of bread”. This explains why Duo won't accept “he wants another bread”. That said, “he wants another loaf” isn't an appropriate translation for “er will weiteres Brot”, because here “Brot” is clearly being used as an uncountable noun and the translation should be ”he wants more bread”. “He wants another bread” is instead “er will ein weiteres Brot”
“He doesn't want another bread” is certainly an error on the part of duolingo, although the unmarked translation would be the one analysing “Brot” as a mass noun, i.e.: “he doesn't want (any) more bread”. However, “keines” could also be a negation of the indefinite article, making the translation ”he doesn't want another loaf (of bread)”. You should report the latter as an acceptable translation, but remember that “er will kein weiteres Brot” can be (and more often is) the negation of “er will weiteres Brot” rather than “er will ein weiteres Brot”.
‘Brot’ in German can be used both as an uncountable (‘bread’) and countable (‘loaf (of bread)’) noun, which means that depending on the context different translations are needed. Without knowing exactly what you wrote, I can't tell you whether ‘loaf’ was appropriate here, but in this particular sentence both should generally be ok (‘he doesn't want any more bread/another loaf’). In a sentence where ‘Brot’ is preceded by the indefinite article, only ‘loaf’ would work, whereas in a sentence where ‘Brot’ has no article at all, only ‘bread’ is possible.
When you write your answer, Duo will look in the database for one that closely resembles yours to give you the most appropriate correction. This means that if your answer deviated significantly from the one suggested here, Duo might have pulled a different correct translation (notice that getting the gender of the article wrong, for example, is enough for Duolingo to go looking for a different translation, so ‘significantly’ is used very liberally here).