I think you’ve figured out that the ACTPHY01 table doesn’t accept nulls for B1N4CD field, and that is the reason why you tried to use “coalesce” function in the second command, is that correct?
I think you are using COALESCE in wrong way, because COALESCE uses 2 parameters, the first one is for the field that you know has null values, and the second one is for the value that will be returned everytime that a null value in found in the specified field.
In other words you must specify new value to replace nulls from the orginal table using COALESCE function, like this:
UPDATE DABCON2/ACTPHY01 A SET A.B1N4CD = (SELECT COALESCE(B.B1N4CD,0) FROM DABCON2/VDA_CS B WHERE A.B1LWNB = B.B1LWNB)
I’m assuming that B.B1N4CD field is a number, change the zero value in the second parameter according to your needs.
Tell us if this works for you.