YouTube 'not available' for your domain
If you've seen the following message, it means that YouTube isn't currently enabled as a service on G Suite:
YouTube is not available for domain.com. Learn more about Google products you can use with firstname.lastname@example.org. Did you use this product with a different Google Account? Sign out of your current Google Account and then sign in to the account you want.
How to add YouTube as a service
Only your domain administrator can enable YouTube on your domain. Let the domain administrator know you'd like them to turn on the ability to add YouTube as a service. Once they've enabled this, you should be able to add a YouTube channel directly to G Suite.
If you're the domain administrator, find out how to add services to your G Suite domain.
YouTube can't be enabled for my domain
If you're in a country where YouTube has not yet officially launched, YouTube unfortunately cannot be enabled on your G Suite domain. You can check which countries are launched by clicking or tapping Account > Location.
You can still create a YouTube channel on a consumer Google Account separate from G Suite.