Search a few owners forums for similar problems, a mis-diagnosis can be expensive.

Can’t really offer any help but I can tell you that these are notoriously unreliable gearboxes and should be avoided at all costs when buying used especially the dry clutch type as fitted here. Most warranty firms charge double trade premiums to cover them which tells you all you need to know. I’d offload it quickly rather than spending money on it and replace it with a much more robust torque converter gearbox or a simple manual.