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 replace except with excluding, like this:
All of my friends wanted to spend Friday afternoon at the
beach. Excluding Michael, the worrier, who was
concerned about shark bites, jellyfish stings, and skin cancer.
Unfortunately, both except and excluding can begin afterthought fragments. Since there is no main clause in the part about Michael, you still have a fragment on your hands. Review the rules.
Go back to the sentence to try again.