You need to change the expression, so to check for empty, you need to use a comparison. Try the following...

$TaskToValidate/DueDate = empty or $TaskToValidate/DueDate >= [%CurrentDateTime%]

I hope this helps.

I'm trying this process:

MF_ValidateTask (Validation Before Saving)

Ensures a non-empty title and a future date.

In App Explorer, right-click Microflows > Add Microflow.

Name: MF_ValidateTask > OK.

Drag Parameter (lightning bolt icon) > Name: $TaskToValidate, Type: Task > OK.

Drag Exclusive Split (decision):

Click on the decision > Properties > Expression:

$TaskToValidate/Title != empty

Branch No connects to:

Show message > Message: Task title is required! > OK.

End event to abort.

From the Yes branch, add another Exclusive Split:

Expression:

empty($TaskToValidate/DueDate) or $TaskToValidate/DueDate >= [%CurrentDateTime%]

(It's giving an error because of the '('))

Branch No:

Show message > The due date must be today or in the future.

End event. Final Yes branch:

Change Object > Object: $TaskToValidate > set nothing (just passes straight through).

Show message > Validation ok!

End event.

Save the microflow