Take a look at the passage again:
As he scrubbed the tile grout with a toothbrush and bleach. Nick vowed never
again to let the bathroom get so filthy.
You chose to move the main clause to the front, like this:
Nick vowed never again to let the bathroom get so
filthy, as he scrubbed the tile grout with
a toothbrush and bleach.
Unfortunately, you do not use a comma to connect a subordinate clause to the end of a main clause. Review the rules.
Go back to the sentence to try again.