You may already well know the Azure Multi Factor (MFA) and Azure Self Service Password Reset (SSPR) features; if not you are missing a very good piece of your Azure AD/Office 365 services.
Well, a new registration experience for Azure MFA and Azure SSPR is currently being in preview to simply the end-user registration experience. With the current experience, end-users had to register twice their details (like phone number or secondary email): once for Azure MFA and once for Azure SSPR.
With the new experience, when they register for one of the service (either MFA or SSPR), their details is automatically shared with the other service.
If you want to start using the new experience, you must enable your organization for it through the Azure AD portal (either from the Azure portal https://portal.azure.com or the Azure AD portal https://aad.portal.azure.com)
Go to the Users Settings configuration blade
Then go to Manage access panel settings
Finally define how the new experience will be enabled: either to selected users/groups or for all users
Once it is done, the new/preview registration experience will be used
The below example of the new registration experience; the user registers for MFA and define his phone number, and after the initial verification the same phone number is then defined for the SSPR
Off course the other way around (register for SSPR first) will do the same 
Are you participating in the private preview? I get “You are not enabled to manage this setting”.
Hi
this is not a private preview; this has been publicly announced
I had the same error for the past few days and it sorted by itself
You can always try to raise a service request to O365/Azure AD support