If on a question I click close → Off topic because... → This question belongs on another site in the Stack Exchange network, I get just one option: Ask Ubuntu Meta.
How do I migrate, for example, to Unix & Linux?
Ultimately you can't vote to migrate a question anywhere but meta. There are so called "migration paths" that would enable slots in the close dialog, but these "paths" have been closed for quite a while due to misuse of the feature in the early days of the site.
Previous attempts to have them opened have met with moderator resistance.