This error means there is some other license already assigned to the user, and one of the service plan from that license rely on the F3 license that you are trying to remove.
You can check the user and see if there is any direct/other group base license assigned to that user.
One of the service plan from that license will be dependent on the F3 license service plan that you are trying to remove.
Let me know if you have any further questions.
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