Take a look at the original sentence again:
Lorna did not last long in Mr. Wolcott's busy office,
her long fingernails made accurate typing impossible, and her abrasive
manner scared away too many potential clients.
You wanted to fix it this way:
Lorna did not last long in Mr. Wolcott's busy office,
because long fingernails made accurate typing impossible, and her
abrasive manner scared away too many potential clients.
Adding because, a subordinate conjunction, in front of her long fingernails made accurate typing impossible creates a subordinate clause. You can, in fact, fix a comma splice by subordinating one of the main clauses. However, when a subordinate clause follows a main clause, you generally do not use a comma. The comma in front of because makes this choice wrong.
You might want to consult the rules for fixing comma splices and fused sentences.
Go back to the sentence to try again.