Here’s what the Flux documentation says about creating a PCB board (board outline) in the PCB view:

“In this tutorial, we'll explore the different ways to create and modify board outlines in Flux, allowing you to customize your PCB shape and size to fit your project requirements.

Overview

The board outline defines the physical boundaries of your PCB and is one of the first design decisions you'll make. Flux provides several options for creating board outlines, from simple shapes to complex custom designs.

We'll cover the following topics:

Basic Shapes

Flux has built-in support for simple shapes (circular and rectangular). For complex shapes, please refer to the section on Advanced (custom) shapes.

Changing the Outline Shape”