I am trying to buy a house, there is no water to flush the toilet and no water comes out of the bathtub faucet, but there is water in the sink, what could be the reason?

Possible shut off valve somewhere in the line. Tubs don't usually have stops.

How long has the house been empty, assuming it is.

Have you considered hiring a professional to look and give you a estimate,

Or consider the worst, need new lines to those, and ask for a price reduction to cover repair costs, Or even ask seller to fix this before you close.