runtime error 91

You never assigned a range to your rng variable.

1) This code is probably in a sheet module - please mention such details (or better attach sample file).

2) When code first calls

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rng variable is not assigned (Nothing)
and in function RdmCell you get error 91 because code tries to count cells in not assigned Range Type variable.

May be it would be enough if you preassign rng before entering a loop like:

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