You Cannot Play on Realms because of how your Microsoft Account is Set up Nintendo Switch?

Why Can't I Join Realms on Switch?

Players under 13 years old will have online multiplayer disabled by default with Minecraft and won't be able to join Realms or servers unless a parent changes their account settings. For help with changing these account settings, you will need to change your Privacy and Online settings of the Xbox Live account.

How do I Enable Realms on Microsoft Account?

Log in to Xbox settings with the parent Microsoft account .
Select the child accounts profile.
Select Xbox One/Windows 10 Online and choose Allow for Join multiplayer games AND You can create and join clubs first picture below.

How do you Get Realms to Work on a Switch?

Now that you've signed in to your Microsoft account, select Play on the home screen.
Toggle to the right onto the Friends tab.
Select the button to Add Friend.