Take a look at the passage again:
Despite Cassandra's stern warning, Alex walked over to flirt with Helen.
The most beautiful—and dangerous—woman at the party.
You chose to connect the appositive to the main clause in front, like this:
Despite Cassandra's stern warning, Alex walked over to flirt with
Helen the most beautiful—and dangerous—woman
at the party.
Unfortunately, you cannot remove the period and think that you have taken care of the problem. Review the rules.
Go back to the sentence to try again.