"I work as a secretary at an office."
Translation:Ik werk als secretaresse op een kantoor.
The een before secretaris is optional. However it could be that you were penalised for using secretaris. Secretaris and secretaresse refer to two different functions. Secretaris is an administrative function and can be used for both males and females, secretaresse is a personal assistant and is normally only used for females (for males usually personal assistant, persoonlijke assistent or administratief medewerker is used). See this explanation (in Dutch).