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Hi I'm Brian and I'm here to help!
The early Rheem control boards did not have a landing for the Y connection directly on the control board. I suspect yours is one of them. In this case, the wire connected at Y at the the thermostat will connect at the air handler directly to one of the two wires going to the outdoor unit with a wire nut (or one of the many wires going to the outdoor unit on a heat pump). The other wire on a straight cooling system going to the outdoor unit would connect to C on the control board.
That would be the answer to your question, but I suspect you are having a related issue here. Is there something with the low voltage wiring I can help you with?