In the sentence: "Before his death at age 82, comedian Rodney Dangerfield's motto was, 'I don't get no respect.'", what is the grammatical error?

The grammatical error in the sentence lies in the use of "no" within the phrase "I don't get no respect." This construction is an example of a double negative, which is considered grammatically incorrect in Standard English. Instead, the phrase should use "any" to maintain proper negation, resulting in "I don't get any respect."

The rest of the sentence is structured correctly, with "was" serving as the linking verb that connects the subject (Rodney Dangerfield's motto) to the complement (the quoted statement), "at" correctly indicating the time of his death, and "before" setting the temporal context for the entire phrase.