Item group ID missing for variants

Variants of the same product (different colors, sizes, materials) need a shared 'item_group_id' to group them; missing IDs cause variants to be treated as duplicates.

Microsoft Merchant Center uses identifiers (GTIN, MPN, brand, SKU) to match catalog records. Issues come from invalid format (non-numeric in a GTIN field), brand-GTIN mismatches in GS1's database, duplicate IDs across SKUs, or missing required identifiers for the category.

1. Verify GTIN ownership using GS1 GEPIR (https://gepir.gs1.org) — confirms which brand legitimately owns the code.
2. Strip non-numeric characters from GTINs: no dashes, no letters, no spaces.
3. Check ID lengths: UPC = 12 digits, EAN-13 = 13 digits, ISBN-13 = 13 digits starting 978/979.
4. If your product is private-label without a real GTIN, request the channel's GTIN exemption process rather than fabricating one.
5. Re-submit; new identifiers take 24–72 hours to propagate.

• Buying GTINs from non-GS1 resellers — usually maps to defunct brands and triggers validation
• Reusing the same GTIN across multipack and single-unit variants — flagged as duplicate