Firtly SRID задать этот запрос 4326 использовать его;
SELECT ST_SetSRID(ST_MakePoint(-71.1043443253471, 42.3150676015829),4326);
SELECT P.*
FROM PRODUCTS AS P
LEFT JOIN MERCHANTS AS M ON (M.MERCHANT_ID = P.STORE_ID)
LEFT JOIN CATEGORIES AS C ON (C.CATEGORIES_NAME = P.CATEGORIES)
LEFT JOIN LOCATIONS AS LO ON (LO.LOCATION_ID = 20)
WHERE P.CATEGORIES ILIKE ANY(ARRAY['Cameras']) AND
ST_DWithin(M.LOCATION::geography, ST_AsText(ST_SetSRID(ST_MakePoint(-71.1043443253471, 42.3150676015829),4326))
GROUP BY P.PRODUCT_ID, M.MERCHANT_ID