DIAGNOSIS The siren is a separate little speaker inside the camera, and this error means the system can't activate it. It's often caused by a recent firmware update that didn't apply cleanly or a temporary software glitch.
FIX The standard reboot usually fixes it. In the Arlo app, go to Settings > My Devices > select your camera > Restart. Wait two minutes for it to come fully back online. Then, test the siren by going to the live view and tapping the siren icon.
IF THAT DIDN'T WORK You need to check the siren settings themselves. In the device settings, go to Siren Settings. Make sure the volume slider isn't at zero and that the siren is enabled for your current Mode (like 'Armed'). Also, make sure a schedule isn't silently disabling it.
HEADS UP If a reboot and settings check don't work, it's likely a firmware bug. Check for another update. If the siren is permanently dead, it's a hardware fault and you'll need to contact Arlo support, as the siren isn't a user-serviceable part.