A parent user is assign when a user is created/invited into the portal by customer person with the appropriate permission for IAM in the IBM account.
That person with the IAM permission can do anything except disable MFA that the user enable by himself in the portal.
It doesn't make sense that you are responsible for managing that user, but you cannot disable MFA which is required when the person cannot log in. Than IBM support tell you to reach out to the master account owner or another IBM team responsible for IBMid support.
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