2000 Buick Century getting low coolent flash on dashboard

Anytime coolant is disappearing, but there's no evidence of a leak externally, then it's an internal leak.

Unfortunately, this means you have a blown head gasket. There's only two places coolant can disappear to. Either on the ground (External), or inside the motor (Internal).

Where the head gasket blew will dictate where the coolant is going. Pull your oil dip-stick. If the oil is white (Like a milkshake), then that's your coolant. If it's clear, then your coolant is going into the cylinders and out of the exhaust.

Fortunately, your engine isn't very expensive to repair. But if you've blown a head gasket, I'd say that engine isn't really worth saving. Buy another one. They're still pretty cheap. Just shop around until you get a good price.