the condition must match the name of the condition on eBay and must be available in the selected category.
Can you double-check if the condition “Used” does exist in the selected category (and if the category does exist)?
Before importing the file it might help to update the eBay category data first:
Open the GarageSale preferences > Accounts. Click on “Refresh Token” to refresh your eBay access token.
If done, select a listing and open the category browser. In that browser click on the sprocket icon to update the category data.