If you want users to select any category, then you need to use JS and PHP code.
JavaScript Hook example: add this to any javascript file.
wp.hooks.addFilter('dokan_middle_category_selection', 'dokan', function () {
return true;
});And add this to your functions.php file,
PHP hook example:
add_filter( 'dokan_middle_category_selection', 'dokan_middle_category_selection' );
function dokan_middle_category_selection() {
return true;
}