Save the settings form of the screen that is open, with its "Save Changes" button, and wait for WordPress's answer: the notice "Settings saved." or "Permalink structure updated." It fails with the error notice when WordPress shows one.
Examples
When I save the settings
When we save the settings
And I save the settings
When we save the changes
And I save the changes
Pattern
When(/^(I |we )*save the (?:settings|changes)$/, …)
Part of @webship-js/wordpress-steps (github.com/webship/wordpress-steps), a step pack for Webship-js 2.0.x.