She’ll start barking eventually, give it time. Soon enough the problem will be getting her to shut up LOL.
Once she does, with my two I started by giving the command when they started barking on their own for whatever reason and praised. After doing that some so they got the general idea, I started giving the command during play. This is gonna sound silly but it worked for me, I’d give the command and start barking myself
they’d get worked up enough to bark back.
After doing that some, I stopped barking myself and just gave the command during play and refused to let them have the toy till they barked. Didn’t take long till they figured it out.
After that, I randomly did it with no toy but with a treat handy. not offering though as in a 1:1 bark to treat exchange, rather you’re to bark because I told you to. good job, here’s a hidden treat.
ETA: I still use it regularly during play, it’s great for getting drives back up and getting their “head back in the game” so to speak. It’s a great tool to have, major deterrent. Saying a word in a foreign language and having a GSD bark on command, has an amazing power to make people rethink their behavior. Lol