Hi VivienneReis,
Thank you for using the Apple Support Communities! We understand your iPod Classic is alerting you to use iTunes to restore the device, however, once connected, the device does not appear to be recognized. We'd like to help.
If you're running into the same behavior, try seeing if using a different cable to connect your iPod Classic to your computer helps. If the same thing happens with a different cable, you could try the steps found in this support article: If your computer doesn't recognize your iPhone, iPad, or iPod - Apple Support
If you are unable to resolve this behavior, please feel free to reply back and let us know if you are using iTunes on a PC, or Mac. If using a PC, let us know which Windows version you're using. If using a Mac, let us know which macOS you're using.
Best regards.