"A spider is an animal."
Translation:עכביש הוא חיה.
You need a copula here to connect the two nouns because otherwise you don't have a sentence but a two-noun construction. Hope that helps.
In older phases of Hebrew you could find עכביש, חיה הוא, but not in modern Hebrew.
a word in the answer "hu" is not in the word bank
i.e. same as above: The word "he" was missing from my options so I changed to the keyboard to type it in. I tried to report it, but the missing word wasn't an option.