Вам нужно добавить все остальные столбцы, кроме агрегированных столбцов, в группу по пункту
SELECT auctions.auctionId, quantity, startPrice, reservePrice, startTime,
endTime, itemName, itemBrand, itemDescription, items.image, auctions.views,
item_categories.categoryName as subCategoryName, superCategoryName,
item_categories.superCategoryId, item_categories.categoryId, users.username as sellerUsername,
conditionName, countryName, auction_watches.watchId, COUNT(DISTINCT (bids.bidId)) AS numBids,
MAX(bids.bidPrice) AS highestBid,
case
when MAX(bids.bidPrice)is not null THEN MAX(bids.bidPrice)
else startPrice
end as currentPrice,
case
when MAX(bids.bidPrice) > auctions.reservePrice AND auctions.endTime < now() then 1
else 0
end as sold
FROM auctions
LEFT OUTER JOIN bids ON bids.auctionId = auctions.auctionId
JOIN auction_watches ON auction_watches.auctionId = auctions.auctionId
JOIN items ON items.itemId = auctions.itemId
JOIN users ON items.userId = users.userId
JOIN item_categories ON items.categoryId = item_categories.categoryId
JOIN super_item_categories ON item_categories.superCategoryId = super_item_categories.superCategoryId
JOIN item_conditions ON items.conditionId = item_conditions.conditionId
JOIN countries ON users.countryId = countries.countryId
WHERE auction_watches.watchId IN( 19706, 19707 )
GROUP BY auctions.auctionId, quantity, startPrice, reservePrice, startTime,
endTime, itemName, itemBrand, itemDescription, items.image, auctions.views,
item_categories.categoryName as subCategoryName, superCategoryName,
item_categories.superCategoryId, item_categories.categoryId, users.username as sellerUsername,
conditionName, countryName, auction_watches.watchId
ORDER BY CASE WHEN auctions.endTime > now() THEN 0 ELSE 1 END ASC, auctions.endTime ASC