MySQL #1241 - Operand should contain 1 column(s)

Question

I want to update the column 'page_inv', so it will replace two last chars of this column, and add new data, for example:

'{"userItems":[{item1}]}' -> '{"userItems":[{item1}' -> '{"userItems":[{item1}, {item2}]}'

UPDATE `users` 
SET `page_inv`=CONCAT((SELECT `page_inv`, SUBSTRING( `page_inv`, 1, CHAR_LENGTH( `page_inv` )) -2), ',{newItem}]}') 
WHERE `STEAMID`=76561198147

My DB:

Answer 1

Something like this?

UPDATE `users`
    SET page_inv= CONCAT(left(page_inv, length(page_inv) - 2), ',{newItem}]}') 
    WHERE STEAMID = 76561198147;