Here is the original item:
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.
Adding 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.