Where is the clutch slave cylinder on a 1992 Nissan sentra

grating geras in a manual especially first and reverse ae an indication of a clutch problem
If you have not already replaced the clutch slave cylinder and clutch master cylinder ( clutch cable if so equipped) then that are the first things to check
normally it is the slave cylinder is leaking and will not hold pressure or the master cylinder rubbers cannot develop enough pressure to fully disengage the clutch
( cable breaking or stretched does the same thing)
If there are no other apparent problems with the clutch ( slipping , burning smell, shudders etc I would pass on a clutch kit until it is a last resort

You have not indicated whether your car is a stick shift or an automatic transmission.
If it is a stick-shift, look for a problem with the master cylinder or the slave cylinder on the clutch.
The clutch is actuated by hydraulics, and loss of brake fluid from the system will cause the car to stop shifting.
The master cylinder for the clutch is smaller than the brake master cylinder, and is mounted under the hood (in front of your knees while you are driving).
The slave cylinder is mounted on the transmission near the clutch assembly.
More often than not, the slave cylinder fails before the master cylinder.

If your transmission is an automatic, and the fluid is correct on the transmission dipstick, you may need a transmission overhaul, which can be very expensive.
God bless your efforts.

Start with lubricating the clutch pedal pivot up under the dash. Then check the hydraulic components in the system. There is a flexible rubber hose that connects the clutch slave cylinder mounted on the transmission case to the metal tubing from the clutch master cylinder. The inside of the hose can deteriorate internally without leaking to the point where it will pinch off or restrict the fluid flow which causes a stiff pedal.

low fluid indicates a loss of fluid so top it up Check the slave cylinder for leaks at the piston . If ok replace the clutch master cylinder as the compensating port is blocking and not allowing the fluid to return to the reservoir and is holding the clutch off.

Check your clutch master cylinder's fluid,it's located under hood in right corner,if low or empty,then your slave cylinder is bad and leaking,it's connected to side of transmission.Here's a link. Repair Guides Clutch Clutch Release Cylinder AutoZone com Here's the part. Duralast Clutch Slave Cylinder 12425 1992 Nissan Datsun D21 Truck 2WD 4...

bleed it at the slave cylinder ,open the nipple. pump the clutch then need someone to help you .or use the self bleeder so hold the clutch going down call out pump it. down and hold so going down nipple open .hold nipple closed.

you need to bleed the cluch slave cylinder located on the transmission and the clutch operating cylinder located in the passenger front hood compartment next to the power steering resivoir

ON A 2000 MAX 6 CYLINDER HAS CLUTCH MASTER CYLINDER AND SLAVE CYLINDER, CHECK FLUID LEVEL AND BLEED CLUTCH SYSTEM AT SLAVE CYLINDER BLEEDER. HAVE NOT HEAR OF CABLE CLUTCH SYSTEM. ONLY HYDO.& LINKAGE. GOOD LUCK

Most likly needs a new clutch. but before you do that make shore the clutch slave cylinder is releasing.Do this by opening the bleed screw on the clutch slave cylinder. If there is preasure released when you do his you may have a stuck master cylinder. Not likly but worth the trouble to try

If your not able to shift into any gear that means your clutch is not engage/disengaging. Check for fluid leaks, you said topped off and fluid levels were low? It sounds like you have a Slave cylinder or master cylinder leak, if its leaking its bet to replace both.