Twain biggest fault when he worked as a printer was that he

Leaving the prospects of what being a printer could have made him.

Explanation:

Mark Twain the author is can be summarised as a man of contradictions, who has multiple pen name as befitting his multiple personalities and quite a complicated person.

He at the ages of 18 to 22 he worked as a printer in his brother Orion's paper traveling from St. Louis, Missouri to New York, Cincinnati and Philadelphia

His fault as a printer and editorial assistant was that he did want to write comics and satire not his brother's choice of literary job nor those of St. Louis's so he left the course and the prospects of being a printer to become a river pilot boat.