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:

  1. Attempt to power off the console.
  2. If the console will not power off, hold the Power Button for 15 seconds.
  3. Power the console back on, and continue with the setup process, except skip the process of linking a Nintendo Account.
  4. Once the setup has been complete, update the console to the latest system menu version.
  5. 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.