How do I run more than one brand?

Each brand has its own name, link, look, homepage and galleries, sharing one account, plan and storage.

Open the brand menu at the top of the sidebar (at the top of the screen on a phone) and choose New brand….
Type the Studio name and Your link, then press Create brand.
Switch between brands from the same menu: the one you're managing has a ✓.

Why isn’t it working?

It says your plan's brands are all in use: Your plan's brands are taken: each one more is $5 a month. See Plan & storage › Plans.