Actually, you have a much more important grammar point to take care of; it's in your title. To form questions in English, we invert the subject and verb, so that the verb comes before the subject: Why is this sentence incorrect?

@Benoit Selignac

You're right about your example, but not about it being 'the only context' : that's by no means the only context where 'courage' wouldn't have an article. In fact, it never has an article, unless you're referring to the kind of courage which is specific to a particular situation. Unlike in Latin languages, 'courage' as a concept - along with love, hate, fear, jealousy or all other abstract qualities - has no article in English.

So why is it OK in Guisela's example? It's because the object of the sentence is the entire noun phrase 'the courage to do such a thing'. Here, we're not talking about 'courage' in general, but about the specific type or amount of courage needed to face that particular thing. This is the exception to the rule - the only context where an abstract concept requires an article.

It just needs "a" before "thing". Countable nouns in the singular always need a determiner in English. Spanish has the same rule, except that in Spanish the indefinite article is not used before "otro" or after "tal". That's what's causing your problem; you're translating from Spanish "tal cosa". Try thinking of it as "una cosa así".

It is not incorrect, it is just lacking any preceding context this phrase always follows preceding context.

First you need to mention something that requires or needed courage::: then you can say 'You need the courage to do such a thing'

But

'You need to have the courage to do such a thing'

'You need to find/pluck up the courage to do such a thing'

'You need to have courage to do such a thing'

'You need to find/pluck up courage to do such a thing'

are the most common ways to use the phrase and leaving out "the" is much more common.

Such is a determiner referring back to the thing just mentioned that requires you to use 'a' in that phrase.

@Karl, there's missing context in this sentence for sure, but "you need the courage to..." is a very common phrase. Maybe not "to do such a thing", but rather concrete actions such as "~ to speak up" or whatever.

I could see it working in some text where "such a thing" has already been mentioned. But if you just want to say courage in general, then sure, I'd agree and say "courage" and not "the courage".