Birdy Builders, is a Quacktopia minigame. It is Quacktopia's version of Build Battle, where you are made to build a theme that will be shown to all users. There was an agreement you signed not to build anything inappropriate, if you did, it would result in a ban.
Sadly, Birdy Builders was shut down, the sign to access the minigame is still there although this doesn't work, nor does the command. This could be due a bug when updating to 1.13.2.