Hi,
When the engine is off, there is nothing preventing it from shifting. Once it's started, then the clutch separating the transmission from a moving flywheel is needed. That is why it won't shift.
When you had the transmission out, did you replace the pressure plate, clutch disc, and release bearing? See pic 1 below. Also, did you use a clutch alignment tool to align the clutch disc, flywheel, and pressure plate cover before tightening the pressure plate?
Let me know.
Joe
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Friday, February 18th, 2022 AT 4:03 PM