Here is the original item:
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 this subordinate clause to the end of the main clause. Review the rules.