Complex sentence: The proclamation outlawed private purchase of Native American land, which had often created problems in the past; instead, all future land purchases were to be made by Crown officials "at some public Meeting or Assembly of the said Indians".
Split into simpler sentences: The proclamation outlawed the private purchase of Native American land, which had often created problems in the past. Instead, all future land purchases were to be made by Crown officials "at some public Meeting or Assembly of the said Indians".