You may find 'Machine identities' and 'Domain credentials' options missing in Machines catalog wizard:



You may expect following options:

According to studio's behavior if you want to add machines for those accounts, you can directly add them without all steps/options like in first screenshot. But if you want to add a new machine which has 0 available accounts, then you'll have to create an AD account and then machine which follows steps from second screenshot. 

OR  Increase the 'Number of machines to add:'

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Problem Cause

The catalog have more than 0 available accounts. Meaning the catalog already as the AD accounts created in the domain.

machine catalog that won't show the two pages, because the NumAvailableMachineAccounts > 0 in the machine catalog, the available machines can directly be added into machine catalog and no need to create new machine. So this is the expected behaviour.