With the new Facebook policies taking effect on March 4th, only Pages that are registered with the Facebook News Page Index (NPI) will be allowed to send non-promotional subscription messages.
To submit your page for review, go to your page settings> Advanced Messaging.
Then, under Requested Features > Request (Next to Subscription Messaging)
Fill out the questionnaire and submit.
Once Facebook approves your page, you will be able to use the 'Subscription' messaging tag in your Re-Engage tab.
It's highly important that relevant pages submit their application immediately to register with the News Page Index to allow sufficient time for review and approval before the new policies go into effect.