During the first time setup process, the console stays on a preparing screen when you attempt to link or create a Nintendo Account.
Complete These Steps:
- Attempt to power off the console.
- If the console will not power off, hold the Power Button for 15 seconds.
- Power the console back on, and continue with the setup process, except skip the process of linking a Nintendo Account.
- Once the setup has been complete, update the console to the latest system menu version.
- After the system is updated, return to the user settings and link your Nintendo Account.
I recently picked up a Nintendo Switch for Zelda: Breath of the Wild. When setting it up and linking my Nintendo Account, the screen was stuck on "Preparing Device" or something "Preparing." I waited ten (10) minutes and found this article.