Review the original item:
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.