My cortisol 24h urine result was 68nmol/d and the range was 0 - 193nmol/d how is it possible for zero cortisol to be within the normal range?

Diagnosis of Cushing: A 24 hr urine cortisol is used to help in the diagnosis of cushings syndrome (high cortisol). It is a useless test for the diagnosis of low cortisol (addison's disease). The normal range is really to indicate where the upper limit of normal is, not the lower limit. Your level was normal, and therefore not consistent with hypercortisolism.