Hi @Bruno Caruso ,
Thanks for your post! Multiple subscriptions can trust the same Azure AD directory, but each subscription can only trust a single directory. When you create a B2C tenant, you need to do so within the same directory that contains your subscription(s). This is documented here.
You indicate that you tried to link the the tenant to a different directory since you say that you can see the subscription once you switch to your root tenant, so you will need to add the B2C in the root tenant in order to add the subscription. Otherwise you can transfer the subscription to a different Azure Active Directory or add a new subscription to the B2C tenant.
To link a subscription to an existing Azure AD B2C tenant, you need to:
1) Switch Directories to the location of your target Azure subscription
2) Under Marketplace, search for and select 'B2C'
3) Select Create to link this B2C Tenant to a subscription
Let me know if this helps and if you have further questions.
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