Duolingo likes ''hence'', ''therefore for ''daher'' but says "thus" is wrong. I do not see any difference in English.
Same with "ergo"
Ergo is Latin not English
The first time I was asked to translate this sentence, 'thus' was given in the correct answers when I missed. The second time around, when I tried to use it, it was taken as a mistake. Frustrating is the least I can say.