Final answer:
The error in the sentence is the incorrect verb 'have', which should be 'has' to match the singular subject 'The Newberg Environmental Association'. The corrected sentence would be: The Newberg Environmental Association has worked for decades with one clearly defined goal: to reduce the negative impact of human activity on the natural world.
Step-by-step explanation:
The error with subject-verb agreement in the sentence 'The Newberg Environmental Association have worked for decades with one clearly defined goal: to reduce the negative impact of human activity on the natural world' is the use of the verb 'have' when it should be 'has'. The subject, 'The Newberg Environmental Association', is a singular noun representing a single entity, so the correct verb should be 'has'.
Therefore, the sentence should read: 'The Newberg Environmental Association has worked for decades with one clearly defined goal: to reduce the negative impact of human activity on the natural world.'