The header row contains a column name Shopify does not know. This usually happens when a supplier template was used instead of Shopify's own export.
Always start from a Shopify export, not from a supplier sheet.
The free Shopify CSV checker names the exact line for each problem. The paid Rescue Kit adds 20 more error cases, a template in Shopify's column order, and the same checker as a script for your pipeline.
See the Rescue Kit →