Complex sentence: Upon his return to Rome, he was made a prelate, but when he was denied a nuciature, he temporarily retired from 1717 (after his father's death) to 1721 on his estate in Fossombrone.
Split into simpler sentences: Upon his return to Rome, he was made a prelate. However, when he was denied a nuciature, he temporarily retired from 1717 (after his father's death) to 1721 on his estate in Fossombrone.