I had a customer who called about a single user had issues with setting MFA up to use text, Phone call or even Microsoft Authenticator via. http://aka.ms/MFASetup. The call or text message was never received. In the Authenticator App, when they scanned the QR code, they got the following error pop up:
“Activation failed. Make sure that push notifications are enabled on the phone and your Activation Code is not wrong, expired or formerly used.”
The user was blocked from setting up MFA in the Azure portal. To get him unblocked, go to Azure Portal -> Azure Active Directory -> ( -> Security) MFA
Click Block/unblock users and click unblock on the blocked user. They can now successfully setup the Authenticator App.