MySQL Error 1054: Unknown Column 'hotels.postal_code' in 'on clause'

Question 1

Replace

FROM
malls

        join
    hotel_sales ON hotels.postal_code = hotel_sales.sales_id

AS

FROM
malls

        join
    hotel_sales ON malls.postal_code = hotel_sales.sales_id

Joining MALLS and HOTEL_SALES with right column...

So the final Query would be..

SELECT

    hotels.postal_code AS ' Hotel Postal Code',
    name AS 'Hotel Name',
    latitude AS 'Latitude',
    longitude AS 'Longitude',
    address AS 'Hotel Address',
    hyperlink AS 'Hotel Hyperlink',
    hotels.district AS 'Hotel District',
    'hotel' AS type

FROM
    hotels

        join
    hotel_sales ON hotels.postal_code = hotel_sales.sales_id
        join
    postal_code_location ON hotels.district = postal_code_location.district 

UNION SELECT 

    malls.postal_code AS ' Mall Postal Code',
    name AS 'Mall Name',
    latitude AS 'Latitude',
    longitude AS 'Longitude',
    address AS 'Mall Address',
    hyperlink AS 'Mall Hyperlink',
    malls.district AS 'Mall District',
    'mall' AS type
FROM
malls

        join
    hotel_sales ON malls.postal_code = hotel_sales.sales_id
        join
    postal_code_location ON hotels.district = postal_code_location.district

Question 2

This error is about

Disambiguate column_id in your Query

It should be something like this

SELECT
    hotels.postal_code AS ' Hotel Postal Code',
    hotels.name AS 'Hotel Name',
    hotels.latitude AS 'Latitude',
....