When not running it goes in all gears but when running it's like the clutch isn't engaging
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Do you mean that it's like it isn't disengaging? If the clutch pedal is pushed to the floor, the clutch is disengaged. When you press the pedal to the floor, you've disengaged the clutch, allowing the engine to spin independently of the transmission. If your foot is off of the clutch, the clutch is engaged and able to transfer torque to the wheels.
With that said, if your clutch hydraulics have air in the lines or the slave or master cylinder is failing you may not be able to completely disengage the clutch meaning that with the car running you can't prevent the engine from spinning up the transmission. If this is happening there's a fair chance that selecting a gear from a stop is going to be difficult to impossible.
Basically what's happening is when the car is running and stationary you can't select a gear because you can't get the engine side of the transmission to stop spinning to mesh the shifter parts with the differential side of the transmission that's not spinning.
If you want to test this... with the car off put it in first gear (disengage the parking brake), and with he clutch pedal pushed to the floor start the car. If there's even the slightest jerk or feeling of forward momentum like the car wants to drive off with the clutch pedal completely depressed, or if when you start to pull your foot off the clutch it feels like the engagement point is super close to the floor, you have a hydraulic issue.