Turret not working?

I've just unlocked Rafa's Insta-Fort Battle System that enables the Peacebreaker Cannons to be set down as a stationary turret. The prompt to deploy the turret once the cannons are firing is L1. The icon appears on the screen, but when I press L1 to deploy the turret the icon flashes red and there's a beep to suggest it can't be done.

What am I missing? How do you deploy the turret?