Thank you for your post!
I understand that you're trying to link your B2C tenant to your subscription but when walking through the Create new B2C Tenant or Link to existing Tenant menu options, your B2C tenant isn't populating and you're running into a "no available items" message.
To hopefully help resolve your issue or point you in the right direction - when it comes to B2C tenant's, you should've linked your newly created B2C tenant to your subscription during the creation process. To see this, switch to your B2C tenant and navigate to the Azure AD B2C Overview page.
If you don't have a Subscription linked to your B2C tenant you'll see a message notifying you, along with the Subscription Status showing "No subscription".
Note: If you're seeing the "No subscription" status within your B2C tenant, along with the "no available items" when trying to link your subscription within your Azure AD tenant, please let me know.
I hope this helps!
If you have any other questions, please let me know. Thank you for your time and patience throughout this issue.
If the information helped address your question, please Accept the answer. This will help us and also improve searchability for others in the community who might be researching similar information.