This is because of blank lines and comments you have between your php code. Those will have to be removed.
Look at the first line when you get these errors, where it says "output started at". That is where they are caused... so for this one /add-product.php line 43
You will see you have comments and blank lines between your php code above the <html> tag between the php tags: ?><?php
This is not allowed and will cause header errors when you try to redirect to another page in code below it.