where condition should identify the calendar item that you previously selected on the form. it has to identify just SINGLE calendar item, preferrably by item ID, not all the items with status 'Frei'
on the current item, the workflow is running on, you've populated/selected a lookup field from calendar list.
so you have to take that field's value and search for matching item in calendar list. the best/most reliable would be to search by item ID, but nintex by default search for match by 'value' field. in my example bellow I used Title field. if you used a different field you'll need to customize it accordingly.
so the update action might look like