Hi Den & Kutty,
Please check below solution.
Goto T code OH13 ( Wage type characteristics), Find out your wage type and go for details
Probably you are taking "+" operator in Number/unit in input combination which is not applicable applicable for the wage type.
change it to -( Minus) operator or applicable combinations of input as below
Now check by maintaining IT0008,
Hope it will help you.
Thanks and regards
Pramod Sharma