MFA Error External User Accessing SharePoint

[RMD2021]

An external user who has been invited to access an internal SharePoint site is coming across an MFA error trying to access the site - any ideas why and how to resolve? The user isn't receiving a call but does receive the verification code but after inputting still gets this message.

External sharing is enabled and the users phone number is correct.

Error Code: 500121

[#2]

Not much we can advice you here...best you can do is review that the MFA policies applied to the user are the right ones....if that's the case, open a support ticket