Error: The result of the Selection Formula must be a Boolean
This message is received when saving a formula in the Select Expert where the result of the formula is not a True/False value.
The Select Expert formula must return a true/false value. This formula is used to determine whether or not the record will display in the report.
If the formula needs to return a different value such as Gift Amount or ID, this is not the correct place for the formula. You would need to go to the Field Explorer (View > Field Explorer) and create a new Formula Field from there.