Cascade based on multi select option

Dear Colleagues

I am Thulasizwe from South Africa.

I am developing an ODK form and I have a question where multiple province can be selected and below that question I have question about districts visited in each province. My proble is that I want district option show only the districts of the selected province eg Eastern Cape and KwaZulu-Natal selected then the districts in KwaZulu Nata and Easten Cape for the multi sect district option. Please select. I tried use the follow on filter_choices as follows "selected(${provinces},filter)" but it is not showing the list of districts.
I get this error


Can you please attach the XLSForm?

I think this forms solve the same issue: Multiple choice in cascade - #4 by Grzesiek2010