"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".