My 2000 bravada don.t pass smog they said that computer is not ready i make 70 miles and is still not ready

Your vehicle's engine computer has monitors , need a scan tool ,code reader to see which one's haven't set . Could have code's stored that prevent the monitors from setting .
https://www.obdautodoctor.com/scantool-garage/obd-readiness-monitors-explained
If no codes are stored ,you need to do a drive cycle to get the monitors to set to ready . Doesn't have to be a problem with the catalytic converters .
http://obdii.com/DRIVECYCLE.HTML

I expect the CAT isn't lighting up. After a relatively short time of normal driving the closed loop fuel control should produce the ideal conditions to promote the chemical process in the CAT that causes a sudden rise in temperature that begins the conversion process of reducing the harmful and polluting content of the exhaust gas and that is monitored by the post-CAT oxygen sensor.

The electronic brain needs to "see" that reduction in harmful emissions when compared to the pre-CAT oxygen sensor and when it does it should be ready for test.

I expect the CAT isn't working and while they don't last forever, their ruin can be hastened by all sorts of things from mechanical impact or being quenched in cold water (deep puddles), an oil burning engine, too much fuel, a leaky head gasket or merely old and tired sensors.
Your car has reached the sort of age when the overhaul should begin with a fuel pressure test and a fuel injector test...