Thank you for reaching out to the community forum!
Could you please confirm below.
Have you Enable the registration campaign policy?
If you have Conditional Access policies, please review Conditional Access Policies and check the targeted users is restricted to MFA.
If the users in the targeted group have already registered for MFA it will not prompt to register again.
Hope this helps. Do let us know if you any further queries.
Thanks,
Akhilesh.
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