You can only use this, with the charger connected.
I use this to watch movies on my tv. In the living room it is Oke, because the MacBook is on my desk, connected with a long hdmi cable.
When I decide to watch in the bedroom, it’s a hassle to take the charger with me. 😂
I don’t know why this is not possible without charger, probably to save the battery , and I understand or don’t mind it really but : first world problem