In the FAQ on https://support.google.com/a/answer/6082718?hl=en ... it says:
"Q: I don’t see the option for Inbox by Gmail in Google Apps > Settings for Gmail > Advanced settings. Why not?"
A: Inbox for Work is only available to Google Apps for Work domains that have received an invite. To request an invite, send an email to [email protected]."
I did this weeks ago. Still no answer.