The missing relative pronoun in the sentence 'the people were at the concert had a wonderful time' is 'who' or 'that'. Inserting either 'The people who were at the concert had a wonderful time' or 'The people that were at the concert had a wonderful time' completes the sentence.
The sentence “the people were at the concert had a wonderful time” is missing a relative pronoun to introduce the relative clause. To correct the sentence, you can insert “who” or “that” after “the people.” Using “who” would explicitly refer to the people, while “that” could also work but is more flexible and can refer to things as well as people.
Thus, the corrected sentence should read: “The people who were at the concert had a wonderful time” or “The people that were at the concert had a wonderful time.” These pronouns are essential to link the subject “the people” with the verb “had,” thereby providing a clear and complete thought.