Translation:Hugo can't live without electronic devices.
Ah. I'd just assumed that you were translating what you read into French. In the dictation exercise, it should definitely be accepted since the singular and plural sound the same. These are still pretty new lessons, so they still haven't collected all of the possible right answers. I hope you reported it.
It would be interesting to understand if the singular would just sound weird in French, and that that's the reason that it's plural. In English, it would sound more specific - as in - Hugo can't live without a device in his hand or very close at hand.
My reasoning was flawed anyway, because even if a person owns only one device or no devices, you could say of them that they can't live without electronic devices in the plural: They don't necessarily have to own them to use them or to feel like they needed them.