The part he is referring to...
Could only be the clutch slave hydraulic cylinder. When this part goes out you cannot engage the clutch.
A few things to look out for:
I doubt NAPA carries the part, and if they do, it's not $20. I would expect a price more like $75 from vendors like Vick Autosport, Midwest Bayless, et.
When the slave goes out, often the master is on it's way out or already gone. The master is probably about the same cost as the slave, but is more difficult to replace (it's under the dash above the clutch pedal).
Once the entire system has been fitted with good working parts, bleeding the hydraulic system can be problematic. Nothing that many here have not already been through, and we can certainly guide you through it if you want. But it can be frustrating.
Pete