I see nothing wrong since it is the same answer I gave. It is a new sentence so they just don't have their database complete. I will report it.
"Uso las piernas para jugar (al) fútbol" is the correct sentence, the above one sounds unnatural.
Grammar websites list "jugar" as requiring "a" in this configuration only. Verb + a + noun. https://www.lawlessspanish.com/grammar/verbs/verbs-with-a/
From what I have read this rule was not observed in US/CA/SA. This combined with the influence of the English equivalent without a preposition "to play football" led the RAE to accept the omission of the "a" as an Americanism. Spanishdict gives some good examples of both here. http://www.spanishdict.com/translate/jugar
El fútbol = soccer = English football.
El fútbol americano = American football.
Context sometimes overrules the need for the adjective when it is clear that American football is being discussed.