Rename the columns the checker lists to the exact spelling shown. Delete trailing spaces in the header row. If your spreadsheet saved it with semicolons, save again as "CSV UTF-8 (comma delimited)".

Shopify import error: "Invalid CSV header: missing headers"

What it means: The first line of the file does not match Shopify's column names. Column names are case-sensitive, so "title" is not "Title". Extra spaces at the end of a name, a repeated column, a missing required column, or a file separated by semicolons instead of commas all cause it.

Find the rows in your file

Shopify product import

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The product CSV you are about to import into Shopify, or the one Shopify just rejected.

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How to fix it

Rename the columns the checker lists to the exact spelling shown.
Delete trailing spaces in the header row.
If your spreadsheet saved it with semicolons, save again as "CSV UTF-8 (comma delimited)".

Example

A file with this problem

title,URL handle,Price 
Cotton Tee,cotton-tee,20.00

What the checker says about it

Line 1: Header "title" should be written "Title". Shopify column headers are case-sensitive.
Line 1: Column 3 header "Price " has extra spaces. Shopify needs header names to match exactly ("Invalid CSV header: missing headers").

Once the listed rows are fixed, check the file again; the checker also looks for every other problem Shopify documents, so you do not trade one error for the next.

Questions

What does "Invalid CSV header: missing headers" mean in Shopify?: The first line of the file does not match Shopify's column names. Column names are case-sensitive, so "title" is not "Title". Extra spaces at the end of a name, a repeated column, a missing required column, or a file separated by semicolons instead of commas all cause it.
How do I fix it?: Rename the columns the checker lists to the exact spelling shown. Delete trailing spaces in the header row. If your spreadsheet saved it with semicolons, save again as "CSV UTF-8 (comma delimited)".

Other Shopify import errors

Sources

Shopify Help Center: Importing products with a CSV file: https://help.shopify.com/en/manual/products/import-export/import-products
Shopify Help Center: Using CSV files: https://help.shopify.com/en/manual/products/import-export/using-csv
Shopify Help Center: Solutions to common product CSV import problems: https://help.shopify.com/en/manual/products/import-export/common-import-issues

Based on Shopify's help pages for product CSV import. Not tested on a live store.