Cannot add categories: headers already sent

Ok so after looking a bit into the code, your constants are not defined (as the notices say). Those notices cause php to trigger the warning because headers are sent for the notice…

What are those constants? Can’t you just remove those?

I have made some changes to your plugin to remove those notices. These are patches, so you should look into your code as to why you are using undefined constants.

You can add these lines to your plugin in index.php after your constant definitions I have defined the missing ones:

define( 'WP_CATALOGUE', plugin_dir_url( __FILE__ ) );
define( 'WP_CATALOGUE_PRODUCTS', WP_CATALOGUE.'products'  );
define( 'WP_CATALOGUE_INCLUDES', WP_CATALOGUE.'includes'  );
define( 'WP_CATALOGUE_CSS', WP_CATALOGUE_INCLUDES.'/css'  );
define( 'WP_CATALOGUE_JS', WP_CATALOGUE_INCLUDES.'/js'  );
//these were used but missing, please try to replace these with useful ones
define( 'WPC_SCRIPT', 'WPC_SCRIPT');
define( 'WPC_STYLE', 'WPC_STYLE');
define( 'WPCACHEHOME', WP_CATALOGUE);

Also there is an undefined index in
/wp-catalogue/products/wpc-product.php on line 78

to avoid this I have added an isset() check like this:

// verify this came from the our screen and with proper authorization,
    // because save_post can be triggered at other times
    if ( isset($_POST['itemmeta_noncename'])
        &&?false == wp_verify_nonce( $_POST['itemmeta_noncename'], plugin_basename(__FILE__) )) {
    return $post->ID;
    }

Also you are using $_POST contents even when they are not set, so you should better guard your code like this:

// OK, we're authenticated: we need to find and save the data
    // We'll put it into an array to make it easier to loop though.
    $item_meta = array_intersect_key($_POST, array_flip(array('product_img1',
                                                              'product_img2',
                                                              'product_img3',
                                                              'product_price')));

instead of this in /wp-catalogue/products/wpc-product.php on line 84

// OK, we're authenticated: we need to find and save the data
    // We'll put it into an array to make it easier to loop though.
	$item_meta['product_img1'] 		= $_POST['product_img1'];
	$item_meta['product_img2'] 		= $_POST['product_img2'];
	$item_meta['product_img3'] 		= $_POST['product_img3'];
	$item_meta['product_price'] 	= $_POST['product_price'];

this way when there is no post content you avoid the notices.

Thanks for reporting these fixes – very much appreciate your time on it. Will check it out for next version ASAP.

Thanks again.
Maeve