i buying 1993 blazer the guy said it don't pass smog what would be the problem

Hello, I can't speculate on the cause since I have no data for the vehicle, but there are very many reasons to fail smog, and professional diagnosis is needed. If it were me, I would require a professional inspection and diagnosis before purchasing. A "good deal" on a nice car, truck, or SUV is not worth it if the repairs are the same price as the vehicle. Californians have been getting ripped off on vehicles like this since smog checks were instituted.