i have a question about english grammar!!

brazil is not only the most populous of the twelve country but also the largest in area

why 'in area' has to be corrected?

It is an ambiguous error, unfortunately.  Readers can identify the error, but will not be able to offer a precise correction without more information.  Sorry

Do you mean it is "the largest in terms of area." = in other words, it is the biggest of the 12 countries
Do you mean it is "the largest in the area." = it is the largest of the countries in close proximetry , for example in South America.
There may be other plausable corrections too.
 

i have a question about english grammar!!

brazil is not only the most populous of the twelve country but also the largest in area

why 'in area' has to be corrected?

It must be IN THE AREA because you are talking about a specific area already mentioned.(South America)