How to check if datetime equal tomorrow in MySQL

You can use the following solution, using DATEDIFF and DATE_ADD:

SELECT * 
FROM sales_order 
WHERE DATEDIFF(created_at, DATE_ADD(CURDATE(), INTERVAL 1 DAY)) = 0;

or a simpler solution only using DATEDIFF:

SELECT * 
FROM sales_order 
WHERE DATEDIFF(created_at, CURDATE()) = 1

DATEDIFF() returns expr1 − expr2 expressed as a value in days from one date to the other. expr1 and expr2 are date or date-and-time expressions. Only the date parts of the values are used in the calculation. - from MySQL docs.