Why my answer 'I need a soap' is wrong? There is only one soap so there should be written 'a' article in this sentence.
I think, your answer should be acceptable in Indian flavored English. In India, when we say 'a soap', it means a bathing bar / a bar of soap.
Otherwise, soap is a substance and found in liquid form also. Hence, it is an uncountable noun.
Why my answer i need the soap is wrong
I need a soap