Q: I can't shift into any gear

It sounds like you may have a throwout bearing that is hanging up. When the car is not running, nothing within the transmission is spinning, thus it is possible to put the car in gear. With the engine running, there is too much friction in the throwout bearing, resulting in your inability to shift into gear.

You may need to replace the following: 1. Clutch 2. Pressure plate 3. Throwout bearing 4. Flush fluid through slave cylinder

If the slave cylinder fluid flush does not solve the issue after the clutch overhaul, a new slave cylinder should be considered next. I would suggest having a professional from YourMechanic come to your location to take a look at your vehicle to properly diagnose and repair.