This is one of those sentences where I'd expect us to switch to a yw/ydy form. "[pro]noun is an other[pro]noun."
The yw/ydy only works in the third person.
This sentence could be turned around to 'aelod o'r staff dw i' and is accepted as an alternative in this exercise.
Since 'aelod o'r staff' is not really a job title rather just a description of someone's relationship with a group the emphatic sentence is less appropriate.
eg Beth yw'ch gwaith chi? (What's your job?) - Athro dw i (I'm a teacher - lit: Teacher I am)
Ble dych chi'n gweithio? (Where do you work) - Dw i'n gweithio yn y banc (I work in the bank), dw i'n aelod o'r staff (I'm a member of the staff)