Take a look at the item again:
Darryl has the messiest book bag (A)
of anyone Rachel knows; (B)
he essentially has a compost pile rotting in the main
compartment. (C)
For instance, pistachio shells, pencil shavings, and a brown banana.
You wanted to do this:
Darryl has the messiest book bag, of anyone
Rachel knows; he essentially has a compost pile rotting in the main compartment.
For instance, pistachio shells, pencil shavings, and a brown banana.
A comma after bag creates an error. Of anyone is a prepositional phrase. When you connect a phrase of this type to the end of a main clause, you need no punctuation.
You might want to review the rules.
Go back to the sentence to try again.