Similes in English take a standard form of " as xxx as yyy" (so although "like" is a perfectly good word it doesn't sit comfortably in the pattern).
Thinking about your question: as a native speaker of English, I would normally use an article here. So "As sour as a lemon" (or "as lemons"... but the Hebrew isn't plural in this sentence)
I have to admit I'm not sure WHY just saying "lemon" as a sort of abstract flavour doesn't sound right, but it doesn't. Perhaps because similes are often using something that's "concrete" to describe something that's not?
I wrote "as lemon" but after reading your post I have the answer for both of us. "as sour as lemon" refers to the flavor of lemon, which can be ver sour, as in lemon juice, or not very sour if there is a lot more sugar than acid. "as sour as a lemon" refers to the fruit (presumably without any sugar added) which is indeed very sour.
I think my answer better explains why "as lemon" should not be accepted here, even tho it IS indeed grammatically correct. "as" without an article can be used in many expressions in perfectly good English: "it's as good as gold" "his muscles are as strong as steel" 'that tastes as good as chocolate." "ice cream is as exciting as sex" In all these cases, the second term is an uncountable, i.e. you could not relplace the word with its plural form: "as good as golds" "as strong as steels" etc. (sometimes referred to as collective nouns). The word lemon has both acceptations in English: countable when it refers to a oiece of fruit, uncountable when it refers to the flavor. (Cf. an orange (a fruit) vs orange (a flavor,... or an adjective meaning the color orange: "during the height of the fires, the sky in California turned orange."
In British English the usual translation would be AS sour AS a lemon. The absence of a second "as" in vocabulary provided may be because this course leans to American English and there are small, but significant, differences between British and American usage. Either that or it is a simple programme error. Nothing is perfect in this world and my Israeli Hebrew teacher doesn't agree with all the Hebrew either but, overall, it's pretty good,☺
Either with the indefinite article or without. The latter may be a little less common in American parlance, but works fine for the purpose.
Interestingly, when you buy a secondhand automobile (or any consumer product) and it turns out to have a lot of maintenance issues, you call it "a lemon". In that case you could not omit the indefinitely article. 'That car I bought last weekend is lemon' would not make sense.
a common misconception. They are not interchangeable. "a lemon" is the fruit. "lemon" with no article is the flavor. It can be used in this way as an attributive adjective: "a lemon soda" tastes like lemon. With the marvels of modern industrial chemistry it may have no lemons in it, just a taste that reminds us of lemons. Think of another fruit: an orange. Orange without the article can refer to a color, such as "the president has orange hair" -- with not the slightest bit of fruit in it, of course. Or "an orange sunset"
This is a minefield. In the UK, the spelling "color" would be considered a mistake. We spell it "colour", and we have not even started on the core of this thread yet! Suffice to say that, in an ideal world, the translation programme would cope with both English and American English. I am occasionally thrown by differences that cause perfectly correct translations to be marked as incorrect and the English vocabulary choices unintelligible to an Englishman. I am learning a great deal about the differences between British and American English in daily usage. It is both diverting and informative.