This error happens because Shopify does not support adding variation names to an existing listing.
If this error occurs, your marketplace listing and the Listing Mirror listing likely do not have the same number of variation names. In the example shown below, the marketplace only shows Package Quantity as a variation name whereas the Listing Mirror listing shows two variation names.
To fix this error, you can do an End and relist for each listing. Here's how to do that: End And Relist. It will delete the Shopify listing on the marketplace and recreate it with what is in Listing Mirror, using all the variation names.