In Chinese, there is no such thing as an equivalent to adverb in English. Chinese basically and mainly use adjective, 形容词， to describe everything.
So the Sentence, <他舒服地躺在那里。>, can be rewritten as <他躺在那里很舒服。>， which means you don't need a 地 or 的 at all，which is sort of the -ly equivalent to English. You don't need that. Chinese grammar is easy and not as strict as English grammar. When it makes sense, then it's correct.
On the other hand: <他舒服地躺在那里。> translation to English would be < He is comfortably laying there.> or < He is laying comfortably there.> or < He is laying there comfortably.> While all the above three English sentences are correct, Chinese is not the same.
Why the second one is bad writing? Technically, it's okay, but it's bad. For instance, in English, a bad way to call 中国人 < Chinese > would be < the People of China >，or even worst, < People living in Mainland China >. Why would you use this awkward long phrase when there is a single simple word to describe the same thing?
I will give you another example why you should never use English Grammar as an analogy when trying to learn Chinese or any other Language. For instance, the word "Red", it's an adjective or a noun in English, but in Chinese， 红，can be used as everything.