Hi AITTSV
Generally, that occurs when you have selected the Base Hourly or Base Salary pay item, not the other pay item i.e. the Annual Leave paid pay item. So do ensure that you are not selecting one of those pay items.
If the pay item is not named Base Hourly or Base Salary is the Pay Basis also greyed out? If so, when you look at your Pay item>>Wages list do you have two pay items called "Base Hourly" & "Base Salary"? The reason for that question is that users can change the default pay item name but it will still have its original attributes i.e. it's the default pay item, therefore, you can't change them for this situation as you can't reduce a pay item's hours when you are trying to pay out through that pay item.