"بِخَيْر اَلْحَمْدُ لِله!"
Translation:Well, praise be to God!
Adverb vs. adjective. "I'm doing good," is not correct in English, while "I'm doing well," is. So if the person asked "How are you doing?", then "Good," is not actually a correct answer. Of course, you hear it all the time, so Duo should probably accept it but, if they aren't, then that's the reason.