What job did Papa have at Fort Lincoln, and why did he have that job in Farewell To Manzanar?

You can find the answers to these questions in Chapter 6
of the book.

Mr. Wakatsuki was arrested soon after the
Pearl Harbor attack and was sent to Ft. Lincoln, which was a men only camp for enemy
aliens in North Dakota.

While he was there, Mr. Wakatsuki
worked as an interviewer.  He helped the government interview other of the Issei -- the
first generation Japanese immigrants who were not allowed by law to become
Americans.

He was picked for this job because he was one of
the few men in the camp who was fluent in both Japanese and
English.