CAT_VALIDATION_ERROR_CUSTOM_LABEL_INVALID fires when one of the custom_label_0 through custom_label_4 fields contains content that exceeds Snap's length limit or includes disallowed characters. Custom labels are advertiser-defined values used for product set segmentation.
Snap Catalog Manager's product schema has format and value rules per attribute. This issue is typically: missing a required attribute for the category, an invalid enum value (e.g., 'XL' submitted where 'M, L, XL' is the valid set), out-of-range numbers, or attributes that conflict across the feed.