Brewing can only take place when the Holding Area of Storage is vacant. If you view the top right area of the screen, you can tell whether a batch is in the Holding Area if a batch number is shown next to the "Held:" text:
To move the batch out of the Holding Area, go to Menu > Manage > Storage, and then tap on an empty storage slot to move the batch out of Holding, and then your team can continue.
