Pandaren cannot choose a faction.

[carlarx1 0]

Ever since the 8.3 update, the pandaren were unable to choose they're faction, the option does pop-up but you are unable to pick any faction, you just stay neutral forever.

[Razmataz 14]

This shouldn't have been working properly prior to 8.3 either.

For the time being you can use .char changefaction or .char changerace to resolve it, but we do have intentions to investigate and fix.