Fixing the Plugin

If your theme isn’t already importing the Datepicker plugin from jquery-ui, you’ll need to add wp_enqueue_script( 'jquery-ui-datepicker', false, array(), null, true); that to the theme’s functions.

You also need to save the settings once to initialize all of the required values.

Line 27 of the plugin needs to be changed to $plugin_url=plugins_url().'/simple-calendar-picker'; to make the CSS import successfully.

In order to output the date to the input field, the jQuery commands at line 133 need to be changed to look like this:

jQuery(document).ready(function() {
 jQuery('.calendarpicker').datepicker({
				dateFormat: '<?php echo $date_format;?>',
				changeYear: <?php echo $change_year;?>,
				changeMonth: <?php echo $change_month;?>,
				maxDate:'<?php echo $maxdate?>',
        			onSelect: function(date, inst){
        		    		jQuery('.calendarpicker').val(date);
	        		}
	});
});

https://wordpress.org/plugins/simple-calendar-picker/