You don't need to click the simplify workflow button to add/remove statuses from a board. Assuming they exist in the workflow for the project it's a drag and drop action.
If you are trying to edit the workflow itself, check the project roles/permissions to insure you have access, being a "Project Admin" does not always mean you have access to every project. Also, I would not modify the JIRA simplified workflow it's self, suggest you make a copy and modify that, unless you want to update all existing and future projects.
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