@Size is a Bean Validation annotation that validates that the associated String has a value whose length is bounded by the minimum and maximum values. And as your exception says it does not apply to Integer type.
Use: @Range
@Column(name= "discount_percentage")
@Range(min=0, max=90)
private Integer discountPercentage = 0;
Or you cloud also use only @Max or @Min and that will work too. For more info please take a look on this link.