"Why is he not able to walk?"
Translation:Hvorfor kan han ikke gå?
One typically does not use "at" after modal verbs, such as: kan, vil, må, etc. Otherwise, it is used if there are two verbs. "Jeg elsker at svømme", "jeg plejer at svømme", etc.
This is similar to English: One typically does not write "I will to swim", "I can to swim", etc., while one does write "I try to swim", "I want to swim", "I love to swim".
Have you come across this pdf before? If not, it might help you out. http://basby.dk/modul1/structure_of_a_danish-phrase.pdf