The array for post_category needs to be an array of integers, not a string, and that’s what you are getting from the $_POST[‘cat’] the way you have it written. Change the value in the option to the category id instead.
The array for post_category needs to be an array of integers, not a string, and that’s what you are getting from the $_POST[‘cat’] the way you have it written. Change the value in the option to the category id instead.