If it will go into gear when the engine is off but not when the engine is running, the clutch is not disengaging. It could be the clutch, possibly warped or put in wrong, or it could be the hydraulic clutch mechanism.
The 1986 Honda Prelude does not have a clutch cable. The 1986 Prelude has a hydraulic clutch instead of the clutch cable.
Does a 1998 Honda prelude have a clutch cable
Yes it does
No, slipping is not affected by low hydraulic fluid level. The hydraulics of your clutch only DISENGAGE your clutch, they do not engage it. If it's slipping, it's either worn out, the pressure plate springs have weakened or you have oil on the clutch disk.
You can't aprrently on a hyrdraulic clutch, which these have that adjust automatically.
You have 2 switches A and B above clutch petal under dash
The Manufacturer
85 Lude, old school cable clutch... No hydraulics (fluid)
No drop it from under the car.
You could have air in the lines
Honda Prelude was created in 1978.
Perhaps the problem may be that your Honda's clutch is wearing out.