What is wrong with this sentence Omar and Mary are champion snowboarders Their starring in an upcoming commercial for a national sporting goods store?

The issue with the sentence is the incorrect use of "their" instead of "they're." The correct sentence should be: "Omar and Mary are champion snowboarders. They're starring in an upcoming commercial for a national sporting goods store." "They're" is a contraction for "they are," while "their" is a possessive pronoun indicating ownership.