Translation:That green shirt is more expensive than the yellow one.
Chinmaya, "costly" is used less than "expensive," in my experience. But, if the rest of your sentence was correct (we can't tell because you didn't copy and paste it into your post), Duo should accept it.
Costly may just not be in the database. If that's the issue, just report that "my answer should be accepted."
Camisa and therefore amarilla are feminine but I thought feminine words that began with 'a' used the masculine definite article 'el'? eg el agua.
Edit; answered my own question: https://www.thoughtco.com/substituting-el-for-la-3079094