Final answer:
The error in subject-verb agreement in the given sentence is the use of the plural verb 'are' with the singular subject 'fortress'.
Step-by-step explanation:
The error in subject-verb agreement in the given sentence is the use of the plural verb 'are' with the singular subject 'fortress'.
Since 'fortress' is a singular noun, the correct verb should be 'is' instead of 'are'.
The correct sentence should be: A newly uncovered Viking fortress on Zealand, Denmark's largest island, is shaped like a ring and may have served as a military training ground for Viking attacks on England.