Take a look at the passage again:
All of my friends wanted to spend Friday afternoon at the beach. Except
Michael, the worrier, who was concerned about shark bites, jellyfish stings,
and skin cancer.
You chose to connect the afterthought fragment to the main clause in front, like this:
All of my friends wanted to spend Friday afternoon at the
beach: except Michael, the worrier, who was
concerned about shark bites, jellyfish stings, and skin cancer.
Unfortunately, you cannot connect these two parts with a colon. Review the rules.
Go back to the sentence to try again.