Final answer:
The error with subject-verb agreement is the use of 'were' instead of 'was' for the singular subject 'Each window and door'.
Step-by-step explanation:
The error with subject-verb agreement in the sentence 'Each window and door were removed, sanded, and stained with a dark varnish before the Kellys put their house on the market' is with the verb 'were'. 'Each window and door' is considered a singular subject because 'each' signifies that items are being considered individually. Therefore, the correct subject-verb agreement for this singular subject is 'was'.
The corrected sentence should read: 'Each window and door was removed, sanded, and stained with a dark varnish before the Kellys put their house on the market.'.