Etsy does not accept listings with inventory quantity larger than 999 items or units on any variation. So if you have even a single SKU in a product with a larger stock, Etsy will reject your listing.
This is a common problem because some listings coming from Shopify or from eBay have inventory greater than 999 because sellers list self-made items of which they have virtual unlimited stock.
Solution: Quantity Bulk Edit
You can use Nembol Quantity bulk-edit function to trip the availability of bulks of listings at a time: select your items in Nembol, click the Action menu, select “Quantity bulk edit”, assign 999 or lower quantity, and Save. All your products and all their variations will have inventory stock trimmed to the quantity you pick, and your Etsy listings in error will self-fix.
You can edit your inventory stock quantities on Etsy in bulk, using Nembol (see the screenshot)