Shopify: Option value cannot be blank

A variant row leaves one option empty while others fill it. Shopify cannot build a variant matrix with holes in it.

How to fix it

1. List every option name your product uses (Size, Colour, ...).
2. Make sure every variant row fills every option, even when the value is the same.
3. Do not invent a value like '-' to satisfy the importer; it becomes a real option.
4. Re-import.

How to avoid it next time

Design the matrix before writing the file. Three options means every row carries all three.