As of system software 13.2.0, current for December 2021:
- Go to the Home screen, and tap System Settings
- Scroll to the very bottom, and tap System
- Scroll to the very bottom, and tap Formatting Options
- Tap on Format microSD card
Every guide I read was either wrong, or missed steps. Maybe an update rearranged things, or changed menu text.
Also remember that SD cards aren’t plug-and-play on the Switch, probably because there’s no clean way in the UI to unmount them.