Problem
 

Inventory sync is failing to sync to Etsy with the error message: quantity must be consistent across all products.
 

Solution
 

You must enable the "Quantities Vary" option for any Etsy listings you have with multiple variants.
 

Go to your product listing in Etsy and click on "Edit quantity options" next to the Quantity section. Here you must select the "Quantities Vary" box for all possible variation types. This must be done for all products that have variants on Etsy.
 

Example: