Albert Barnes Commentary Job 16:21

"That he would maintain the right of a man with God, And of a son of man with his neighbor!" — Job 16:21 (ASV)

Oh that one might plead for a man - A more correct rendering of this would be, “Oh that it might be for a man to contend with God;” that is, in a judicial controversy. It is the expression of an earnest desire to carry his cause at once before God, and to be permitted to argue it there. This desire Job had often expressed; see (Job 13:3), note; (Job 13:18–22), notes. On the grammatical construction of the passage, see Rosenmuller.

As a man pleadeth for his neighbour - Hebrew “the son of man;” that is, the offspring of man. Or, rather, as a man contends with his neighbor; as one man may carry on a cause with another. He desired to carry his cause directly before God, and to be permitted to argue the case with him, as one is permitted to maintain an argument with a man; see the notes at (Job 13:20–21).