How to set default value in function

The syntax you use is correct, but as the error message says, the value has to be a compile time constant.

A compile time constant doesn't make sense for DateTime.now().

As a workaround, you can use:

/// Returns `true` is still going on.
///
/// [date] the date to check.
///   as default value `DateTime.now()` is used 
///   if no value or `null` was passed.
bool isOnGoing([DateTime date]) {
    date ??= DateTime.now();
    ...
}