You must be an account owner or admin in order to have the permissions to add/remove users.
If you have the correct permissions to remove a user but are still running into issues, it is likely because of one or more of the following:
- Open incidents assigned to the user—these will need to be resolved before removing the user
- The user is currently on a schedule (or schedules)—they will need to be removed from these before they can be deleted
- The user is directly on an escalation policy—they will need to be removed from the policy before they can be deleted