Jose,
Make sure you are waiting to close the device:
dev = DeviceOpen["Camera"];
snapshots = {};
tsk = RunScheduledTask[AppendTo[snapshots, DeviceRead[dev]], {3, 10}];
RunScheduledTask[DeviceClose[dev], {35}];
or do a Pause[35] before closing the device
Follow this with:
RemoveScheduledTask[ScheduledTasks[]];
Partition[snapshots, 2] // TableForm
This runs for me on a Mac with 11.1.0