I have changed the below code for WP RESIDENCE file property_categories.php from dropdown single select to multiple selection, but now it is not submitting and receiving any values from front end.
Please give a solution.
By adding multiple you allow only for multiple selects on that particular select. But that does not mean it is saved into database with multiple values, or is interpreted as multiple values in the same page.
The code is not working probably because the post variable is now an Array and you need to save it like one. Then you need to check every category and see if is inside the array you just saved and marked as selected. In the same page you should do a print of the variable and see if it saved correctly.