What this usually means is the variant can't be updated at the Inventory Location you've specified in EZ Inventory.

This could happen if the specified location is no longer active or if you're trying to update the inventory in a "standard" Shopify location but the variant is configured to use a "legacy" fulfillment service location for inventory management. This error will also occur if the opposite is true, where you're updating the inventory in a "legacy" fulfillment service location but the variant's inventory is managed by Shopify.

Another reason you could get this error is if the "Track quantity" option is unchecked for the variant: