"I am not Stephanus."
Translation:Ego non sum Stephanus.
I always thought word order in latin is 1) free and 2) mostly SVO. Since 'sum' implies 'ego' the answer 'Stephanus non sum' should be accepted
Latin word order is fairly free. That said, the verb "to be" is a linking verb, and often appears between the things it is linking, like in English.
Most common would be: "(ego) non sum Stephanus"
"Stephanus non sum" is technically correct in Latin, but it would certainly not be the word order used by most Romans.
See my post above. Latin word order is quite flexible, but putting "sum non" isn't very common.