The error mentioned is that Feynman (as PhD supervisor) supplied an important thesis topic to Mano. Mano didn't get the experience of how to evaluate a problem and find motivation within it:

"I accepted [someone else] as a student because he came to me with the problem he wanted to solve. With you I made a mistake, I gave you the problem instead of letting you find your own; and left you with a wrong idea of what is interesting or pleasant or important to work on (namely those problems you see you may do something about)."