Depends how you wrote the sentence. Grandfather's turban, it would definitely be wrong to include the indefinite article. A turban of grandfather's is a poor construction, but might be correct.

Remember, a and the don't exist in Swahili, so whether to add them or not depends on context. As the course is still under construction, there will also be times where you might choose to either use or omit the article and be marked wrong even though your answer is correct. In those instances, report it.