Final answer:
The missing piece of required information from the prescription is the patient's name.
Step-by-step explanation:
The missing piece of required information from the prescription is the patient's name.
The prescription contains the medication name ('Premarin tabs 0.625'), the quantity ('#30'), but it does not include the patient's name which is necessary to identify who the prescription is for.
Without the patient's name, the prescription cannot be dispensed as it is incomplete and unable to be linked to a specific individual.