"Fils" (son) is pronounced FISS
"Fil(s)" (thread/cable/wire) is pronounced FIL
Back-translation "ma souris n'a pas de fil".
The word taught is "sans" and "sans fil" is fixed to mean "wireless".
In English one could say: my mouse is without cord, or my mouse is wireless. Why does Duo not understand: my mouse is without cord? would you need to say.. ma souris est dans cordon?