The work outlines St. Augustine's sinful youth and his conversion to Christianity. It is widely seen as the first Western autobiography ever written, and was an influential model for Christian writers throughout the following 1,000 years, through the Middle Ages. It is not a complete autobiography, as it was written in his early 40s, and he lived long afterwards, producing another important work (City of God). It does, nonetheless, provide an unbroken record of his development of thought and is the most complete record of any single person from the 4th and 5th centuries. It is a significant theological work, featuring spiritual meditations and insights.
In the work St. Augustine writes about how much he regrets having led a sinful and immoral life. He discusses his regrets for following the Manichaean religion and believing in astrology. He writes about Nebridius's role in helping to persuade him that astrology was not only incorrect but evil, and St. Ambrose's role in his conversion to Christianity. The first nine books are autobiographical and the last four are commentary. He shows intense sorrow for his sexual sins, and writes on the importance of sexual morality. The books were written as prayers to God, thus the title, based on the Psalms of David; and it begins with "For Thou hast made us for Thyself and our hearts are restless till they rest in Thee." The work is thought to be divisible into books which symbolize various aspects of the Trinity and trinitarian belief.

Question: Why was St. Augustines autobiography not complete?
Answer:  He wrote it in his 40's. but lived long after that.
Alternative answer: It was written in his early 40's and he went on to write other works