Low fluid would be the place to start but normally a fluid issue will effect other gears and not just reverse.
I suspect the reverse sero that applies the band that engages reverse is the issue. If you can hear it shift into reverse but it doesn't move, then the servo is just not applying the band enough to engage reverse.
I attached the process on how to replace this as I would start with this. Let me know what questions you have.
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Tuesday, January 12th, 2021 AT 4:44 PM