Here's the breakdown:
* Subject: "Peer editing academic papers" (This is a plural noun phrase)
* Verb: "requires" (This is a singular verb)
The issue: A plural subject needs a plural verb.
Corrected sentence:
"Peer editing academic papers require critical-thinking skills and diplomacy."
Explanation:
Since the subject "peer editing academic papers" is plural, the verb should also be plural ("require").