@reference0 must be defined error message when creating parameters in for loop before SQL insert query

Have you looked at the string it produces?

There's no ; on the end of each insert.

First thing I would suggest you try is adding that on:

                query +=
                        @"INSERT INTO waypoint 
                        (booking_id, sequence, address, lat, lng, reference) 
                        VALUES 
                        ((select id FROM booking WHERE reference=@reference" + i + @"),
                        @sequence" + i + @", 
                        @address" + i + @", 
                        @lat" + i + @", 
                        @lng" + i + @", 
                        @reference" + i + ");";

Note the ; added to the last line there.

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