Unity Catalog is configured at the account level, and then you add workspaces into it. In your workspace selector, you should see "Manage account" at the bottom of the list (or go to https://accounts.azuredatabricks.net/). If you can sign in to that, great, navigate to Catalog and this is where you create the metastore. Documentation for that is Create a Unity Catalog metastore - Azure Databricks | Microsoft Learn.
If you can't sign in to the Account console, you may need to set up your Account and assign users to be account admins. You'll need someone who is an Entra ID admin to help with this, documentation starts at Azure Databricks administration introduction - Azure Databricks | Microsoft Learn.
