Why can a meso compound have an enantiomer?

A meso compound CANNOT have an enantiomer.

Let me show you an example.

Notice how if you rotate the right-compound #180^@# CCW on the plane of the screen/paper, it's the same molecule. So they're the same, not enantiomers.

How about these?

Were you fooled? If so, you might want to take a look at some of the following tricks to recognize meso...

These three compounds are in fact the same compound, and they're all meso. Just drawn in different ways.

a) was a Fischer projection. But rotate the top carbon #120^@#, thereby cycling through the substituent positions (#"CH"_3 -> "H", "H"->"OH", "OH"->"CH"_3#) and generating a mirror plane. Whoops, it's meso!

b) was typical line notation with solid and hashed wedges. Rotate the rear by #120^@# (either direction) and you've got a mirror plane. MESO!

c) was a staggered Newman projection. Turn it into an eclipsed conformation and you've got a mirror plane. Definitely meso.

Oh, by the way, all the compounds above were identical. :)

Rotate the first two compounds we looked at from the top of the page on their vertical axes toward you by #60^@#, and you've got the same compound as the bottom few. The #"OH"#'s are now towards you, the hydrogens are still away from you, and the methyls are now on the plane.