Here, Troy is continuing to recount his fight with his boss, Mr. Rand. Troy is fighting for the higher position for himself and all other black workers in the sanitation department. It's interesting that, though Troy feels that blacks are good enough to be drivers, he assumes that they wouldn't be able to handle "paper" or office jobs. You could see this as an example of how racism is so entrenched that black people are a little racist against themselves. Perhaps Troy has bought into the stereotypes that society has forced upon him.
On the other hand, it may just be a practical observation. Many blacks weren't well educated during the time of the play. Troy himself can't read or write and wouldn't be able to do an office job. This, of course, is due to the poverty and racial inequality of America at the time, and is no way related to lack of potential. Either way you look at this comment by Troy, it's evidence of America's legacy of racial inequality.