1999 Chevy Tracker transmission repair

First check the fluid level, if its low add some asap. If the fluid level is fine, look at the fluid is it dark or burnt smelling, feel it does it have metal shavings in it?

It may be worth your while to get a transmission filter change and a flush out of the system. If this doesnt help, the reverse clutch inside the transmission is likely broken and in need of replacement, which is usually more expensive than a rebuilt or used transmission replacement.