What error exists in the phrase: "He had already finished studying at an university when he was called to join the army."?

In the phrase "He had already finished studying at an university when he was called to join the army," the error lies in the use of "an" before "university." The word "university" begins with a consonant sound (the "yoo" sound), which means it should be preceded by "a" instead of "an." The correct phrase should read "at a university." The use of "an" is appropriate for words that start with vowel sounds, while "a" is used for those that begin with consonant sounds. This distinction is important for grammatical accuracy and proper usage in English.