The Category NAME and Category ID are two different things. From what it sounds like, you are trying to add a category name and entering in a category ID that doesn’t exist.
The easiest way to find your category ID is to go to Posts > Categories …..then click on one of the categories. Once the link takes you to the edit screen of the category you have chosen, take a look at the url and the ID is located near the end. For example http://example.com/wp-admin/edit-tags.php?action=edit&taxonomy=category&tag_ID=3&post_type=post – in this link the ID is 3 …..(ID=3) So if we were using this category, we would place “3” in the custom field.