It could one or two things. How much slop do you have in your gear selector? It has bushings that can and do wear out over time. Get under the car and manually shift the car into park at the transmission, then try to start. If it does then your linkage requires attention. If you are positive that it is in park and it still will not start, then more than likely your neutral safety switch needs to be replaced.

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Dave
1983 300D Daily Driver