this means the ECU doesnt know the position of the gear stick, probably the sensor is either drowned with water or battery acid - both which kill it.
Its the triangle shaped sensor on the edge of the transmission where the manual valve (selector cable) is attached to. First check that if its out of adjustment (all indications should light up properly on dashboard and also reverse lights) - then check wiring for corrosion and breaking. The sensor is not that expensive and quite easy to replace too.
Good luck.