Why backbone area can not be stub area in OSPF If there is no ASBR used anywhere in topology. Is it safe to make a backbone area as a stub.

It is not possible to make the backbone area (area 0) a stub area. However, in theory you actually don't need area 0, you could just use a stub area on the whole network. But I would recommend to implement area0, and rather use a stub on rest of the network if you prefer.