You cannot use the value of a variable directly as option="{var: 'Label'}" of a f:form.select. This is a workaround to do it.

TYPO3 Fluid Select Value From Variable List.html

<f:form.select property="sichtbarfuergruppe"
    options="{
	    0: {
			'uid': '{settings.whatEver}',
			'label': 'Vorlesungsplaner und Dozenten'
		},
		1: {
			'uid': '{settings.adminUserGroupId}',
			'label': 'Nur Vorlesungsplaner'
		}}" optionLabelField="label" optionValueField="uid"/>