You can set preferences on the users role. We recommend setting users to automatically use the classic center.

You can do this by Navigate to Setup>Users/Roles>Manage Roles> Open the role that you would like to set this preference for. Then navigate to the preferences tab> Search for use classic interface, check the box and save.

NetSuite dashboard

Now when you assign this role to someone they will automatically see the classic centre when they are logged into NetSuite.