Hi @tanweike and welcome to the Community!
Currently, all columns mentioned in an expression must be present in the dataset. In your case, you can create column [1B] with a dummy value if it didn't exist (see below), before the IF/THEN expression.
There is a more complex workaround. You can use the function columnexists() as described in my answer in another thread here: Optional calculations with columns - #2 by dgudkov