Who makes the final decision on whether a play in baseball is an error or not?

the scorekeeper

This decision is made by the official scorer, as described in Section 10 of the Official Baseball Rules.

The official scorer's role is to make unbiased decision about how each play should be recorded. They typically sit in the press box so that they can review plays from a variety of angles and in slow motion.

The official scorer. sferik is correct.

the fans all vote in a closed ballot

accually the head umpire decides what the call is, not the score keep.